First, with regard to "the plan", you have to understand that with most of these 19th century urban revolts, there usually wasn't exactly a long term strategy in place. Nearly all of them were localized results of anger/frustration/unrest boiling over into violence, which then were either suppressed successfully, or otherwise grew organically into full scale rebellions and or political movements over time. They were extremely fluid and very complex, so any deeper categorical statements about these events aren't really possible, but just bear in mind that there rarely if ever an initial long-term plan in place. The major exception to this was the later revolutions in Austria, but those were outliers in many ways.

Now, as to the barricades themselves, they were, like most such things are, a byproduct of their environment. The key factors in this environment were:

1. Urbanization, and the urban environment. Large population shifts into cities had occurred over the previous several decades, which meant that there was a natural socio-political instability, coupled with a lot of new population, all centered around urban areas. This meant that unrest was often centered in cities, and as such cities tended to be flashpoints.
2. Seats of government and other government assets tended to be located in cities. In contrast to the medieval period where most seats of power were rural (castles), most important government buildings and other such things had relocated to cities during the industrial area. This, coupled with the situation from #1 above, meant that large numbers of potential revolutionaries were in close proximity to important government buildings.
3. Tactical factors. First, fighting in cities in general is very difficult for armies, regardless of era. In addition, during this era specifically, fortifications in general were extremely effective. Armies did not have tanks, engineer vehicles, or mobile heavy artillery with which to reduce hasty fortifications: they had light artillery and small arms, the same as the revolutionaries did. So, building up a barricade in a confined area was a seriously effective way for civilians to contest a trained military force. In addition, most major army garrisons tended to be in rural areas and away from cities, so aside from potential palace guards and security and the like, there wasn't a major military presence in most cities. Armies had to organize outside the city and then assault it, which for reasons discussed above, was a very difficult proposition.

So, the way the "plan" tended to emerge was, a flashpoint would be reached and an armed mob takes to the streets. They surround an area of interest (usually government building(s) in an urban area), then fortify the area around it with the intent of fighting the military force that would eventually be organized and sent in to retake said area. If successful, they then had some leverage to begin making demands of the government, or alternatively, have a base of operations to spread the insurrection more widely.

There are some interesting theories about this. First, it is important to note a few key things: who was revolting, why they were revolting, and who they were revolting against.

1.) In many cases, urban revolutions were led by disgruntled laborers (Marx would call them his proletariat, or perhaps even his lumpenproletariat, depending on their literacy). Few laborers had access to firearms, thus defensible positions were their only hope of persistence. In the absence of weaponry, they had their tools – masonry hammers, chisels, axes, etc. These could be used to dig up cobblestones, break down public works, or otherwise generate the supplies needed to build a makeshift defensible position. The symbolism of the disgruntled mason, chisel and hammer in hand, bestride a barricade, is one of most the enduring 'icons' of western revolutionary history.

2.) Most, if not all revolutions in the mid-century stemmed from the idealism of the French Revolution, meaning that there was a constant hope that sustaining a revolution long enough would cause the state to collapse, or perhaps – as the news of the revolution traveled – a much larger uprising. Thus, longevity was key, and defense would support this.

3.) The revolutionary target was, of course, the state, but the real problem would be the military. Police forces were not yet commonplace in Europe in the mid-century, thus the military was responsible for dealing with rebellion. In most cases, shelling a city with artillery fire was frowned upon (Field-Marshall Radetzky of Austria was notorious for his shelling of Vienna in 1848), thus some element of urban combat was necessary. This played to the advantage of the barricade, as they were often erected in such a way as to force the military to climb them, or they were staggered (so that one would have to go around one to get to another, and so on). See image here.

Barricades were such a notorious and dangerous element of revolutions that many instances of urban renewal in the later-19th century (Paris the key example) were specifically designed around eliminating the utility of the barricade. Haussmann's renovation of central Paris, for example, beautified the city, but it also created large boulevards (for military maneuvering), central avenues (too large for a quick barricade), and macadamized roads (no stones to pull out of the road!). (Side note: many people complained at the time that these new boulevards went right through some of the poorest neighborhoods of Paris, knocking down all tenements in the way in order to do so. This was a very intentional decision, and can be considered a very early form of gentrification.)

In short, these changes minimized the utility of the barricade, thus it became much more difficult to effectively utilize according to the three points above. The barricade was, and is, a symbol of revolution, but its modern efficacy is not commensurate with its mid-nineteenth century utility.

Sources:

• Agony of the Republic: Repression of the Left in Revolutionary France, 1848-51, John Merriman (A fine work, if a bit dry)
• The European Revolutions, 1848-1851, John Sperber (I love this book)
• Insurgent Identities: Class, Community, and Protest in Paris from 1848 to the Commune, Roger Gould (more social history than military, but still quite good)
• The War Against Paris, 1871, Robert Tombs (fantastic step-by-step analysis of the siege of Paris and the abortive Paris Commune)
• The Year of the Barricades: A Journey through 1968, David Caute (gives a nice overview of the legacy of the barricade)
• Napoleon III and the Rebuilding of Paris, David Pinkney (looks extensively at Haussmann's renovation of Paris)

Edit: Added another source and clarified a point.

Disclaimer: I can only speak with authority about barricades in Paris, not elsewhere in Europe.

The erection of barricades was often spontaneous and not accompanied by any central military plan. The basic reasons behind building them are straightforward: they can be easily built out of objects at hand (paving stones, trees, doors, lamp posts, furniture), they prevent the easy movement of soldiers around the city, and they create a fortified position from which to fire on troops and rally revolutionaries in a single spot.

As a military tactic, they were near-useless. In Paris in 1827, 1832, 1834, 1839, 1848, 1852, and 1871 they were quickly overrun and dismantled by the army. On the other hand, they were charged with an enormous symbolic significance. It's important to note that they were built during working-class insurrections triggered by deteriorating economic and insalubrious living conditions for the urban poor, who were at the time being pushed out of Paris by urban renovations pursued by Chabrol de Volvic and Rambuteau between 1812 and 1848. This gentrification, along with increased policing, led to the displacement of Parisian workers from their traditional neighbourhoods (both because the police harassed them to leave newly bourgeois spaces like promenades and boulevards, and because they could no longer afford rents).

The placement of barricades throughout Paris corresponded not to areas which represented the greatest strategic advantage for combat, but to the traditional working-class neighbourhoods of the medieval city centre. They clustered in pockets around Beaubourg, the faubourg Saint-Antoine, and the montagne Saint-Geneviève, a distribution which remained constant over time. In spite of all of Haussmann’s changes to the city, the barricades of 1871 were erected in almost exactly the same locations as in 1848. Revolutionaries were consciously revisiting the urban sites that gentrification had steadily forced them to abandon and reclaiming them as their own. The construction of the barricades was also a literal deconstruction of the city, the working-class pendant to the paving of city streets which had turned them into bourgeois social spaces. In this inversion of the urban order, insurrectionaries involved their fellow citizens in the unmaking of the city by forcing passers-by to lift three or four paving stones each and add them to the barricade before letting them pass. These acts were accompanied by lantern-smashing, a deed which both facilitated combat against the forces of order and represented a symbolic rejection of the system of control their policing function exerted over the city.

Sources:

Auguste Blanqui, Instructions pour une prise d'armes, Cosaques (Grenoble: Éd. Cent pages, 2003).

Bernard Marchand, Paris, histoire d'une ville (XIXe-XXe siècle) (Paris: Éditions du Seuil, 1993).

Testimonies of Jean Baptiste Dupont and Victor Alexandre Bousselin, 28 April 1834, AN CC 586.

Sophie Mosser, “Éclairage urbain : enjeux et instruments d'action” (Paris: Université Paris 8 - Vincennes-St-Denis, 2003).

For more detailed information on barricades, I recommend reading La barricade : histoire d'un objet révolutionnaire by Eric Hazan (recently translated into English as A History of the Barricade) and The Insurgent Barricade by Mark Traugott.