Detailed back-stories are really good for getting to grips with a character, allowing the player to have a history which underlies their actions. A good background makes it clear in the player's mind how the character would react to various situations, and provides motives for acting in certain ways.

A good back-story also provides plenty of hooks for the GM to build on - NPCs in the character's past, events that the character would rather forget about, and so on. All sorts of things that can come up unexpectedly and cause complications in the present.

So make sure your players give you some back-story for their characters!

Transcript

GM: Two scout ships appear in the sky.
Han: Hey, it'sa me, Freddo. Tella Lando it'sa me.
C-3PO: Freddo?
Han: My handle before Greedo. All part of my rich, detailed back-story. It goes all the way back to Alberto!
R2-D2: Let me see that...
GM: No, no, please don't. I've read it.
GM: They fire across your bows.
[SFX]: Pow! Pow!
Han: Hey! It'sa me! Freddo!
Scout ship: We heard you.
[SFX]: Pow! Pow!
Han: It's not just me in here! There are innocent civilians!
{beat}
Scout ship: You will be permitted to land on platform 327.
Han: Why 327?
Scout ship: It's the closest one to the execution chamber.