As I understand it, the portrayal of battles from ancient times to before the gunpowder era generally give way too much credit to swords. It seems that long weapons like spears and pikes were absolutely dominant, not to mention projectile weapons like slings, bows, and crossbows, and then, finally guns and cannons.
So then when we’re swords useful? Were they used often at all? Were they better for home defense maybe or, if they were used in battle, can we assume then that they were reserved for let’s say precarious situations, similar to modern day soldiers and sidearms?
An excellent question, because as you already know, swords are usually given exaggerated importance in pop culture and artistic representations of history.
The short answer is: Personal one-on-one combat. Unless you’re Roman.
To get the Romans out of the way, they of course used a variety of weapons in their military actions. They had archers, mechanical artillery, and several kinds of spear such as pilum or plumbata, but these were mostly for throwing. Yes, technically you can also stab with a pilum but I’ve only found sporadic references to it. However by the time a Roman line had finished hurling everything they could at the enemy from a distance, when they got in close, their weapon of choice was the gladius (and later the spatha). It was short and highly maneuverable in the close ranks of soldiers, all carrying their large scutum for protection. This video from the Smithsonian shows a good example of how they were used even against spear-wielding Gauls. https://www.youtube.com/watch?v=B4HY9u62MBI
When you get to the so called “dark ages” and/or Early Medieval period, swords were prohibitively expensive and so only a limited number of people had them. Generally this was confined to cavalrymen because they were well-off enough to afford horses. The high price is referenced in the Lex Ripuaria in the 7th century, and Regino of Prüm’s work in the 9th century suggests they were predominately seen as cavalry weapons. The Bayeax Tapestry also shows swords mostly being used from horseback, though it also shows most using spears.
One of the earliest sword fighting manuals, an anonymous 14th century work known as simply “I.33” or the Walpurgis Fechtbuch, exclusively shows sword fighting between two unarmored combatants. This work is more trustworthy on depictions of sword fighting in my opinion as the writer is clearly someone who was competent as a fighter (as evidenced by people reproducing the techniques in modern freestyle-sparring). This is as opposed to more fanciful illuminated artwork of the time period, which was often painted by people who were not fighters by any measure, and often were drawing events they didn’t witness. The Walpurgis Fechtbuch depicts the teacher character as a priest, and students (one a woman) drawn in plain civilian clothing, suggesting it was intended for middle class citizens rather than the knightly class. This would suggest sword fighting for personal defense rather than warfare.
Many more detailed combat manuals emerge in the 15th Century, such as those by Fiore de'i Liberi, Hans Talhoffer, and Johannes Liechtenauer. These works mostly deal with “longswords” (a modern term that helps differentiate sword types) which are wielded mostly in two hands but sometimes one. Again, all depictions show one-on-one combat, both with and without armor, and in some cases depict techniques for judicial dueling (aka “trial by combat”). Liberi’s work makes several references to personal self-defense as well, even including techniques for being attacked with weapons while unarmed or when only armed with a stick. Of interest is that Talhoffer shows an unarmored longsword fighter against an armored one, and depicts the unarmored man holding the sword by the blade using the handle to bash his helm in. To me this further adds to the theory that swords were uncommonly use in warfare (at least by this time) because armor made them ineffective without resorting to special techniques.
In 16th and 17th century you start to see the more formal “fencing manuals” especially in Italy. There are too many to list, but they are best described as “rapier dueling manuals” because most of the time, they were intended for unarmored single combat. They depict other weapons as well, though by the 17th century these more or less become sword-only. By this time period, warfare was done with firearms and pikes, and so the sword was a last ditch sidearm for commanders. In civilian life, the sword was mostly a pastime for nobility, but was used in duels as well as self-defense. Michael Hundt, a German fencing master, focuses his work on self-defense, including being attacked by unusual weapons or by multiple aggressors, (where he suggests throwing your sword at them and fleeing).
Works beyond this time gradually morph into fencing for sport, with dueling still happening on rare occasions as well as several works on the use of a sabre for cavalry, for example Alfred Hutton's work during the 1800s. Even so, I’m not aware of that many instances in these later eras were the sabre was actually used for military fighting, other than a few instances which are definitely not the norm.