Ironically, when you first start the owner of the Adventurer's Guild informs you of exactly what you're getting yourself into by saying that helping random people with their problems will "rarely be of realm-shattering importance", but you have to also take it in context.

What sets an adventurer apart from a regular sellsword/mercenary is their potential altruism - adventurers are willing to do things for the greater good without usually thinking of their own well being or even a reward, where as sellswords only do it if there is a substantial paypacket at the end of it. And given the fact Eorzea is still recovering from the Calamity, altruism goes a long way to making Eorzea a better place.

Or at least, that's how I look at it.