Assuming it's on your belt - the short answer is so it doesn't go ker-plunk in the toilet as you're sitting down. When you - how do I put this delicately? - lower your trousers, the grip, normally carried on top, can twist as the belt loosens (it's heavier than the barrel end) and actually end up pointing downward, and if it doesn't fit quite closely enough, the gun can slide out. I know what you're thinking, and no, you cannot make the problem go away by repeatedly flushing the toilet, although a friend of mind did end up disposing of his obnoxious pager that way...
Ok, I get if now. Thanks.