As a librarian, from time to time, it is all about weeding the library. My lecturer would call it part of the collection management. It is essential, unless you have oodles of space.
Books go out of fashion. (A number of management books that are immensely popular for a couple of years or so, and then the library is stuck with them because nobody wants to borrow them) They get outdated (Microsoft Excel 2000 Bible, I am looking at you here.) They end up looking ragged because they’re too popular, or get bad treatment from the patrons – in which case a new copy will be acquired… if the funds are there and if it is deemed relevant.
For my personal library, there are those books I bought because I was bored, and which were fine to relieve boredom for half an hour or so. Or because they looked interesting, and as later turned out, they weren’t.