It grants full administrative access to the phone, letting you run some apps like battery savers, wifi/USB tethering, ad blockers, etc. Lots of nifty things you can do with a rooted phone.
The Nexus and Moto X/Moto G should be the only Android phones worthy of consideration. Everything else is doo-doo :lol:. Those manufacturer UIs and manufacturer specific apps nobody ever uses are battery hogs.