Quote Originally Posted by Athiril View Post
With colour negative overexposing will decrease your saturation....
I always heard that it was the opposite effect. Granted, I've never done rigorous tests, but the thinking is that overexposing puts down more dye in the film, and thus stronger colours. The opposite is true with slide film, where more exposure equals less dye.

At any rate, mark is right about reducing graininess, and that's probably the biggest reason to do it.