A lot of the attraction of a book comes from the design on the cover. When a new edition of a book comes out, it usually features a new cover. Have you ever re-bought a book you already own because you prefer the cover?
I have often been very tempted to do so, especially if the cover I have is not very attractive or in poor condition. I never have though. The only exception is when the new edition has extra content. For example, I bought Peter Pan and The Wizard of Oz a second time because the books also contained the prequel/sequels to the story.
My daughter is making me re-buy books at the moment because her favourite watercolour artist has just done the covers on my favourite author's books. If I wasn't going to read them again I wouldn't do it though.
Since I buy most of my books digitally, this is always a massive temptation. Unfortunately economics has kept me from doing so, but the new hardcover Discworld novels are something that have severely tempted me.