Do you run Windows at home? How about at work?
Have you ever worked at a job where you wanted to install a program but you could not do it because you did not have admin privileges? Perhaps the IT department wanted to make sure they had no headaches due to staff installing Limewire or screensavers with adware or who knows what.
Still, as a techie, you wanted to install Firefox instead of IE or maybe you needed VLC to play back a video file. In that case it might annoy you to not have admin rights.
Getting "root" is the same as having an admin account on your Windows machine. Typically, some system files are locked down either because the carrier wants to artificially prevent you from installing a competing app or because in "noob" hands you could screw up your phone, leading to returns and repairs.
Getting root access means you can do whatever you want to your OS or even install another version altogether. Maybe you want to remove all the Sprint apps you will never use. Maybe you want to use that premium data fee and connect your laptop wirelessly. Maybe you want to take a screenshot. Maybe you want to try out the developer's Froyo ROM that isn't finished testing yet but might be fun.
There are as many reasons to root as there are people who think of these devices as their own pocket computers rather than static appliances that only do what they did on the day you bought them.