The reason is you then have 100% control over your system. If you know what you're doing, having root can be a very powerful tool. You can greatly speed up the phone, free up ram, theme things, it gives you total control.
Now this is a double edged sword too. All the horror stories you're hearing about root are by people who don't have the faintest idea what they're doing and are breaking things.
Think of it in terms of computers. On linux, having root is, well, having root. You have full access to the system. On windows, (vista and above mainly) it's like having an administrator account with UAC turned off. In the right hands, this is very very nice. In the wrong hands, it can cause problems.
If you don't see the benefit of rooting, then I'd highly suggest you do not do so. These devices are not windows boxes where you screw up and "oh well, guess it's time to bust out the windows cd and do a re-install. I'd also suggest perhaps looking through the market at things that require root, and the benefits they give you. (Titanium backup, for instance. Or tethering when your provider doesn't want you to.)