I like GR and track my reading because otherwise I have a tendency to fall for a nice cover and description and then find out I already have the book. Or I've even read it. My reviews are mostly for myself, so I can remember what the dang book is about
That way, when I fall for a book and see I've already read it but don't remember a thing, I can refresh myself, and since I read a lot of sci-fi and fantasy which often comes in series, maybe go buy the sequel instead of the one I just fell for all over again (or not if I didn't like the first one.) I could do this just as well with Calibre, and in fact I do, but it syncs to GR and the app on the phone with the barcode scanner is really convenient in a bookstore (we still have lots of those here) or the library.
I do like the social features and groups when I'm in the mood, and I have some people whose taste matches mine pretty well that I follow for reviews and tips on things to read. Librarian'ing around cleaning the database is a nice relaxing not-too-brain-taxing thing to do when I'm in the mood, or sitting on a train.
But mostly, I just have a shocking memory. GR's great for that.