I like to own books. To have my own copy of the printed page that I can always go back to and re-read. To always be able to referrence some part of something I've read in case I forget. And, of course, because each book is very much like a trophy.
It's not just a "look what I've read" sort of philosphy though. A good part of the effect is to show to whoever might be looking at my bookshelf, "this is what I've deemed worth the time to put into my head," and thus my shelves also convey a large part of who I am (well depending on what shelf you look at. The encyclopedias are there for pragmatic referrence).
But anyway, I enjoy having hardcover books to put on the shelves. They look better and the dust jackets help keep the wrinkles in the spines covered. But I also enjoy having hardcover books for the reading in the first place. And what I really like about hardcovers is that, with the dust jacket, you don't need to have a bookmark. You can just use wrap the jacket over the right pages and always find your place.
And this brings me to why I write this in the first place. Using the dust jacket like this means that you know when you're half way through a book (since you switch from using the front cover, to using the back cover), and that's always a fun moment. And that's what happened two days ago with the first hard cover I'ld read in a while.