There could be many reasons.

For Harry Potter, I have the ones I had as a kid, and then I have the 15th anniversary editions just cuz they’re pretty (don’t come for me, I collected them one at a time all second hand local bookstores, no profiting the terf)

For The Giver, I already owned it but then I found a signed edition at my second hand bookstore so I got another copy

For The Awakening, both of my parents gifted me a copy not knowing the other had.

For Little Women, one of my copies is very beautiful, leather bound with purple engravings, but it only includes part one (the book was originally published as two separate parts and sold as two separate books) and the publisher never published the second part of the book in that same special edition, so I went out and bought a complete version of it so that I can actually read the book in it’s entirety as well as have just a beautiful copy of one of my favorite books

While there are many reasons, one thing that remains consistent is it’s almost always because of second hand purchases. I’d be hard pressed to pay full price to buy a second copy of a book I already own