Gives you complete control of your device.
That being said, some goodies that you can do are
Flash custom hboot, repartition device, read and write to ANY partition on the device, flash boot.img through recovery, flash boot.img, recovery.img, hboot.img, radio.img, almost anything through dd commands in a terminal emulator with su rights.
None of these things are done automagically though, prior knowledge of dd commands is good to have and what is safe and what is not safe are things you'll have to use commonsense and/or Google for.
Personally, after s-off and experimenting with known safe things to do with it, you will gain a lot more knowledge and respect for Android. It's exciting!
I would recommend searching around
www.xda-developers.com
And maybe even joining the community over there.
I have learned all I have to share here, from there.