And let's say that there is an update released that bricks your phone, or some app from the Market borks your phone altogether. Provided you have root access, you can create a backup of your device - all of your apps AND data - that can be moved to an external SD card. When you get the replacement phone, all you have to do is insert the external SD card and, after gaining root access on the new device, restore all of your backed up apps and data without a hitch. Without root, this isn't an option - you could SYNC everything, but that's less helpful, causes you to have to jump through more hoops than that's worth, and generally leads to a much more complicated mess than it has to be.
Rooting is glorious. It's akin to being the Admin of your own computer, versus having someone else you don't know be the Admin of your computer, with you being a limited user, only able to do what they allow you to do, with no input on your part as to what happens or what you're allowed to do. Which is like being in jail, versus being the jailer.