1. Allows installation of third-party ROMs
2. Gives access to the entire device file system, allowing removal of unwanted system apps
3. Allows more comprehensive backup (see above)
4. Allows apps to have low-level access to OS functions, increasing functionality e.g. over/underclocking, location-based profiles
There are no doubt more I've missed - check the stickied Guide at the top of this forum for further insights.