Modified from an earlier answer of mine

The were multiple, overlapping and sometimes contradictory imperatives at work when Napoleon created the Illyrian Provinces in 1809. The main rationale for Napoleon was that the Illyrian Provinces would strengthen the Continental System by putting a large swath of the Adriatic under direct French control. This would strengthen the blockade and end smuggling. Napoleon operated under the general logic that Austria had proven susceptible to British influence and that by eliminating her outlet to the sea, France could gain a more pliant ally on the continent. The French strategic rationale was also that they could create a buffer state in the Balkans on the flank of Southeastern Europe which would act analogous to the Duchy of Warsaw as a check on Austria. If the Franco-Austrian peace remained lasting, the Illyrian Provinces were also a chip Napoleon could return as a reward for Austrian compliance to French-led imperial system. They also could be a direct source for manpower and revenue for French interests in this region of Europe over the short-term. The erection of the Illyrian Provinces was also of a piece with Napoleon's various annexations of other territories like Holland and the Papal States. Unlike other voices within the French government, Napoleon conceived of citizenship as jus soli (from the soil) in which Frenchmen could be made through acculturation to French norms and values. So the Illyrian Provinces fit within Napoleon's conception of the universal French empire he was creating; he was taming allegedly wild areas and making the "civilized" (i.e. French).

That said, the Illyrian Provinces never quite played the role Napoleon intended for them. They were never legally incorporated into the French state via a Senatorial resolution, and thus were in a legal state of limbo. Most of Napoleon's government conceived of citizenship on the basis of jus sanguine (by blood) which precluded any attempt to create a sort of universalized French citizenship. Napoleon himself was reluctant to get France too involved in the Balkans; French reforms were half-hearted and even those yielded meager results. The abolition of serfdom, which would have come if it were a proper component of the French Empire, was not really enforced. The French governors only abolished personal servitude without compensation, but other forms of feudal labor remained intact. Only in the latter half of 1812 did an imperial decree formally abolish serfdom, which was much too late as French fortunes sank in Russia. The French did sponsor some significant road-building and other infrastructural projects, but such internal improvements were more than counterbalanced by the significant damage both the Continental System and cutting off ties from Austria did to the local economy.

The French Emperor would keep prospect of returning the provinces to Austria as a potential inducement to keep Austria on France's side in 1812. Hence, unlike the Low Countries, sections of Italy, or the Rhineland, there was significantly less impetus for Napoleon to push for more modernization as those areas were more integral to Napoleon's vision of an enlarged France. As Governor General, Marmont had relatively free reign compared to the rest of Napoleon's satraps, but he also found himself without significant imperial resources with which to accomplish his task. The weakness of Marmont's administration meant that smuggling flourished and thus its role as an Adriatic cordon against Britain failed. The blockade further ruined the Adriatic economy and added economic hardship to the region, so the French interlude was seldom eulogized by subsequent nationalists on the scale of the Duchy of Warsaw. The Illyrian Provinces thus became a footnote for the larger failures of Napoleon's empire.

The Napoleonic interlude had some impact upon the development of nationalism in Croatia and Slovenia. The French allowed some linguistic freedom of Croatian and Slovene languages within the territory. The French also did develop the infrastructure of the provinces somewhat, such as the "Napoleon's Road" in Croatia. The result was French rule was a relatively benign memory when as direct experiences of the the economic hardships faded and some nationalist activists like Ljudevit Gaj pushed for a Illyrianism and an Illyrian state whose boundaries were roughly that of the earlier French creation. But this nationalist activism was far less wide-spread or articulated than either Poland or Italy. There the Napoleonic interlude left a clearer model for nationalist unification and figures like Poniatowski who could be heroes for the new national movement. While Polish activists like Czartoryski managed to gain some traction in the European public imagination, activists like Gaj did not.

Sources

Broers, Michael. Europe Under Napoleon 1799-1815. London: Arnold, 1996.

Broers, Michael, Peter Hicks, and Agustín Guimerá Ravina. The Napoleonic Empire and the New European Political Culture. Basingstoke, Hampshire : Palgrave Macmillan, 2012.

Dwyer, Philip G. Napoleon and Europe. Harlow, England: Longman, 2001.

Grab, Alexander I. Napoleon and the Transformation of Europe. New York: Palgrave Macmillan, 2003.