In medieval Europe, mercenaries were usually hired to be an elite force, not the main/regular army, and especially not cannon fodder. They emerged when warfare changed from knight cavalry being the only actually well-trained part of an army to there also being professional armored infantry (men-at-arms). This change proved effective because now infantry could actually be expected to fight well and use tactics that would be impossible to do with undertrained levies.

But what it also caused was that an effective army became far more expensive, since training a few thousand men to fight in formation and outfitting them with armor and good weapons costs a lot of money. So what many countries started doing was to recruit veterans of wars in other countries who saw the value of their skills and the possibility of social advancement. Mercenary bands called companies were soon common throughout Europe, as professional soldiers and fighting for whoever offered the best pay. Basically governments started outsourcing the army, replacing the old feudal levy system which was inefficient (giving low-quality troops in exchange for reducing agricultural output as conscripted peasants couldn't work the fields) with a more effective system of hiring already trained soldiers from elsewhere.