When you repeatedly borrow a book from the library, and you hate when you have to return it.
Buy the book if it is going to be a big part of your homeschool.
Educational non-fiction like encyclopedias, histories, atlases, and dictionaries will be used day by day by all ages, and are worth buying.
Buy the books when you find a great deal on books you know you will enjoy and use.
There is no need to buy every book you want or need for your curriculum.
My kids read a lot, there is no way I could buy, and store all the books they go through. The library is our storage place if you will.