I root my phone to install a custom Recovery + custom HBOOT + custom ROM. With custom recovery, I get to make a backup image of my phone similar to Norton Ghost/VSS which is really great in case I decide to fall back. Custom HBoot give me access to change my partition table giving me more space on where I need them and less where I don need them. Custom ROM gives me better usability, better customizations, better stability and whatever the OEMers fail to provide.