Who Needs Money? (DMG)

Treasure is more than just a goal, a measure of material wealth, however. "It takes money to get money," so the old saying goes, and for adventurers one could even say, "It takes money to stay alive." As characters survive and succeed, their challenges become greater and more deadly.
At first level a simple suit of studded armor, a stout pair of boots, and a few simple spells were all a character needed; at higher levels such simple impediments no longer suffice. Faced with terrible foes, characters quickly discover that they need strong armors, barded horses, a variety of weapons, fortifications, men-at-arms, potions, scrolls, and potent magical items.
These are the kinds of things the characters have to find, make, or buy. And however they go about acquiring them, they're going to need money. In a sense, then, treasure is also a method of measuring a character's power. Even a low-level character with money and magic to spare is more than a match for an impoverished fellow of higher level. Thus, getting rich and getting ahead are rewards in and of themselves.