Plunder, booty, and spoils. The military historian Herfried Münkler describes very succinctly how, at a decisive moment in the transition away from Medieval warfare, German mercenaries (Landsknechte) in the 15th/16th c. were practically a landless, roving lower aristocracy that used the army as a legal justification to confiscate goods from defenseless peasants, poorly defended cities, and their defeated enemies. I would highly recommend reading the first several paragraphs of this selection from his book, since you can apply this socioeconomic critical-framework to most military organizations of the Middle Ages.