I generally buy because I live in non-English speaking country and e-libraries didn't really become a thing until recently.

So I buy ebooks that are on monthly offer, so between 0,99€-3,49€. I rarely buy books at full price unless it's something I really want to read (generally books from series). I guess those are the books I would otherwise borrow from the library.

I buy physical copies of the books I read and loved, non-fiction books that I want to have on the shelf, Collins classics and some random books I have gotten as a gift/or bought when visiting book shop because they caught my attention due to interesting description or beautiful cover and were in discount/good price.