What is a Book's Purpose?

The dog gets away with it...really? So, I've been thinking about books lately and their overall purpose. There have been several books I've read in the past few months, where I thought, this part could have been done better or I'd have done it this way (not that I could do it better), but then, I also think, even if I didn't love the book (or even like it), but it brought up emotion in me and made me think about things in some way (even a negative way), did the book do its job? There are certain books I did not love, but I still think about them. To me, so much of our like or dislike for a book is taste, rather than good versus bad writing.
Are there books you've read that maybe you didn't love, but you still think about? Every so often they pop into your head for no reason? Do you think a book's purpose is solely to entertain or to make us think? I suppose, in a perfect world it would be both.
xoxo -- Hilary