My family is pretty big on "useless" decorations around the house. Glass candies, wooden apples, stone angels, a batman lunchbox, a wire sculpture of a hummingbird...
They breath a certain life into the rooms, and are a form of self expression. To that end, I feel like buying a book is almost useless if I don't get to have a physical copy of it to shelve (especially with manga and LNs that are reasonable accessible online for free) and have bought almost none digitally. If I like a story enough to want to own a copy, I like it enough to want to display.
Sure, that's a lot of space taken up by books, which is an issue, especially when I move, but that's a trade I'm willing to make if it means I can read the spine of Peter and the Starcatchers and hear the gravelly voice my dad used for Captain Stache, while reading it before I went to bed, or glance at the cluster of Girls' Last Tour volumes and re-experience the comforting emptiness that it's ending elicted. Seeing the illustrations on the spines of How to Treat Magical Beasts is enough to make my brain whirl, considering the implications of it's magic system. And if I see a book and think "why did I get this one?" then I start to weigh the strengths and weaknesses of the series, and consider what spoke to me about it when I first read it. Even if I only read some of them once, I've smiled at the books countless times.
How much of that is nostalgia and sentimental trite? Good question. There are definitely some on my shelves I could toss next time I need to reorganize them (which I find somewhat enjoyable) but if just glancing at the spine makes my day better, then yeah, I'm keeping them. There's far too many reasons to be unhappy.