I don't know if there's any one thing all users are "supposed" to do on Goodreads or use it for.
I use it to:
1) Catalog my extensive collection. It got to the point that I couldn't remember always what I did or didn't have, especially for old, OP series. Helpful for insurance purposes too.
2) Rate & review books I've read.
3) Interact with like minded readers in several groups
4) Try to get some free giveaways. Gotten a few, but so many people want them, I wouldn't count on it.
5) Keep track of my want list and current reading.
6) I even became a Goodreads librarian, mostly to help correct listings of items from my collection (missing covers, mostly)
Individual mileage may vary.