While I don't advocate for piracy, I also don't advocate for buying the wrong books or hoarding books that you won't use. This is why sometimes having the ability to look at a book before you buy it (same as you would do in an actual book store) is useful.

The "look inside" feature of Amazon is sometimes very limited. Sometimes I've taken a look at PDF or DjVu versions of books, not with the intention of reading them but just looking at them before buying them.

Books approach subjects in varying levels of detail. Sometimes you are interested in deep theory and proofs, sometimes you just want workable formulas.