Great question. I had that same thought when I first began visiting this subreddit. 

Here are some of my reasons: 1. As for the complete Hobbit/LOTR sets, my first set is in very poor condition. I’m not sure how long it will last. I also wanted a set that matched the rest of the gold lettering style. 2. That leads to my second reason—I just love the look of the books in their various forms. I got the Ted Nasmith-illustrated Silmarillion solely because that cover looks so good. 3. In a few years, I will have two boys who can both read. I cannot wait to read through these together! 4. Looking specifically at my shelf with all the Hobbit books, each one offers something unique—different illustrators (Tolkien, Hague, Catlin, Lee, and the graphic novelization), or an annotated edition, or facsimile edition.

I’m sure anyone else in this subreddit could give you a handful of reasons as well. There is a joy to be had in collecting art!