As a game master, you want to enable the group creation of a story. Not utter chaos (all player choices) and not your story (railroad tracks) but the balanced push and pull in the middle. Without you, it would be chaos. Without them it would be drab.
Boiled down to the raw basics, players in tabletop RPGs want to (1) have a unique character in the group and (2) contribute something rewarding to the game. This is the reason that class-based systems continue to be so successful