Rooting is just breaking the protections or locks (however you want to view it) that the carrier puts on it. If you owned a bb previously, custom ROMS are like putting a leaked OS on it. Unlike a leaked OS, which was built by RIM, and leaked out for unofficial testing, Custom ROMS are built by users in the android community. It is cool to try out a few and see if you like them better than what the stock OS is. Also, when your phone becomes older and will no longer get official updates from the carrier, you can count on ROM builders to build you a rom with the lastest versions of Android.
The biggest reason for rooting, IMHO, is the ability to backup my data. Since Android doesn't have anything like Desktop Manager for BB, titanium backup is perfect for me to back up my data once a week in case something happens. You can only do this with Root access as far as I know.