Booking Through Thursday - Why Buy?

I’ve asked, in the past, about whether you more often buy your books, or get them from libraries. What I want to know today, is, WHY BUY?
Even if you are a die-hard fan of the public library system, I’m betting you have at least ONE permanent resident of your bookshelves in your house. I’m betting that no real book-lover can go through life without owning at least one book. So... why that one? What made you buy the books that you actually own, even though your usual preference is to borrow and return them?
If you usually buy your books, tell me why. Why buy instead of borrow? Why shell out your hard-earned dollars for something you could get for free?

While I do like libraries (I work in one), I have to admit that I do buy books quite a bit. I was raised by one super-library-patron and one book-buyer so that may be the reason for my being conflicted.

I guess I like to own copies of certain books for a number of different reasons. I like to have my own copies of books that I know that I'll want to read again (my favorite novels, for example). Knitting books are also must-buys if I like a number of the patterns in the book. I guess the main issue is ready access. There are certain books that I like to be able to get my hands on whenever I want without having to make a trip to the library or place an interlibrary-loan request.