It allows mods, just like a PC does with games, you get access to your phone's inner folders (root) and you can give apps permission to do things they wouldn't be able to do, like change font (other than the predefined), use game killer just like you would use cheat engine on PC, remove pre-installed apps and a couple of other debugging options. It gives you more control over your phone (also makes you slightly more vulnerable)
Generally if you don't have any visual modifications you want to effect, and none of your favourite apps require it, then you don't need it - that's if you're a normal user
If you're a developer or a developer in the making, it allows easy easy installation and Uninstallation of your rom (your OS) and it allows you to edit specific things like the name your phone displays online (you could make it display s6 and playstore and other apps think your using an s6), you can potentially change your resolution too (that never ends well tho from my little experience).
I think I covered almost everything, also if you aren't satisfies with it you can always unroot your phone
OT: how do you know he did not reinstall your exact same OS?
