If you are reading a lot you lose track of what you already read over time. Also, you might not remember how well you enjoyed a particular book and when, if ever, you read it. Still, friends might be asking you for recommendations on a daily basis, and you have a hard time keeping track of which books to endorse and which not.
On the contrary, you might walk past a bookshelf and spot dozens of books you would really love to read but would feel bad if you take all of them home with your right now. Thus, you might scribble down their name somewhere already knowing that you will not remember them anymore.

BookKeeper keeps track of all the books you want to read, are currently reading or read including your personal rating about them.