When you root your phone you can customize it the way you like, you are able to remove preinstalled stock apps that you don't like Ex. Amazonmp3,Nascar,Etc.
Also you can install cool apps that require your phone to be rooted so you can backup data settings etc.
Take screen shoots of your phone to share with friends,
Install the adfree app which blocks almost all the ads that come with the free ad supported apps. and also the add on the internet browser.
Modify or theme your home screens..
there's a lot of stuff you can do.