Three excellent points trendkillah, however I think the main reason why we do so is from mere boredom. After a while, (especially after years of perfecting the board left and right), it's refreshing to be able to change the sound of our guitars, and to express our emotions in any way possible.

The guitar is an amazing instrument, but also, for a lot of us, we are not blessed with such a high range (singing voice.) You'd be surprised how much easier it is to sing sometimes from just tuning the guitar down half a step.

Alot of people feel it's unncessary, but for the most part, for most people it's a defensive reaction. Try it for a couple of weeks, experiment with different settings, just see where it takes you.

-Joseph