I feel like they generally try to serve at least three purposes:
1. Instruction. They've got to teach people how to play/run/design the game.
2. Reference. There're too many rules to really keep in one's head, so they need to be a reference work for the table.
3. Inspiration. Since there's a strong presumption that we'll create our own adventures/worlds/races/classes/spells/&c., it's nice to have a breadth of material presented to make a fledgling GM think "yeah! I *should* do something like that, and I *can* do something like that."