PotterHead there are different ways to use it but the more info you give it, the more it can do for you.
I mostly use it to record what I've read and what I want to read. I don't bother writing reviews but find other people's reviews helpful.
When I tell it what I've read and give the books a star rating. It then makes recommendations based on that.
I also keep a 'want to read' list
You can also tell it which genres you prefer. When you search for books it will offer suggestions of other books by that author, similar books, books that others who read it enjoyed and lists of books that the book appears on. The list might be 'best books of 2019'.
Excuse the dodgy photos of my laptop as some examples of where to find recommendations on the web version. In the app just scroll to the bottom of the page.
In the app you can scan books, either by bar code or the cover to find more info or to quickly add it to a list. Great tool in bookshops etc.
There are also forums, book groups etc which I don't bother with.
I only have 2 IRL friends who use it who I am linked to on there and it shows me what they are reading etc. I can then hit up their bookshelves for a cheeky borrow. It also helps me keep track of which books I have at home so I don't accidentally buy duplicates in charity shops.
You can join the reading challenge and set a target number for the year (editable) and it tells you if you are on track or how you need to pace to meet it.
I find the web version more user friendly than the app for a general browse and when exploring for recommendations.
Basically just click each part to see what's in there and if it suits what you want it to do.