A lot of bloggers have been talking trash about character backgrounds and deriding them lately. While I do agree that they are not 100% needed, they are an incredibly helpful tool for the DM of a sandbox game.
Now, when I'm talking about backgrounds, I'm not talking about the rocks your character has tripped over as a small child, I'm talking about his relationships; his still living family, friends, mentors, and rivals from the days before when he went out to go adventuring.
Think about all the times your players didn't all make it to the game some nights. You could of course just gloss over and pretend the player doesn't exist for the duration of the session, no harm done, or you could mention that a messenger arrived letting the character know something had happened to a family member, and he's needed at home. If your group is on a different plane from home, or too far for the character to travel reasonably within a single session, then possibly a friend or rival made it to where the players were staying and the PC decided that instead of going out adventuring.
Background usefulness doesn't end there. A low level party looking for a henchman instead of a hireling suddenly has an entire roster of people to pick as Henchmen.Those players with living relatives and friends also have a place where they may be able to hide out(even if they have to endure listening to embarrassing stories of their childhood) to recuperate. It's a great place to store wealth in settings that don't have banks, and a useful tool for the DM to provide plot hooks that don't involve a bleedin' inn!