all of the above, plus being able to roll back to an older version.
google was bad about not caring much about backwards compatiblitly for earlier stuff they put out.
so at some point even though the update ran, it started to drag the system down to a crawl so I had to root one to an older build where it still ran ok.
mostly you have to search 2 or 3 sites to find good instructions and compare.
some are pretty good step by step, some have screen shots, some just wall of text, some have you tube videos.
some are out of date by a couple of versions by the time they get them posted, so you have to figure out the step that got added that they missed.
And it depends on how "new" the phone is.
Like rooting on a new samsung 9 might be hard to find instructions that are credible right now. but the 8 or earlier should be pretty easy.
and just like anything look at the review comments on it before you use that instruction set as your base.