People collect them.
I kept the editiones principes (iDISHeeohnays PRINssippeez) of my own books on some shelves in my bedroom until a couple of weeks ago, when a pipe joint popped apart in the roof space and caused a flood down the bedroom wall. Luckily my husband was in the house when the thing burst open, and he, pausing merely to scream in horror (or so I imagine: I wasn't actually there, but the situation was surely worth a scream), heroically advanced through the cascade, seized my dripping Complete Works, and threw them out of the way of the water onto our bed. Then he ran downstairs to turn off the water and hurried back to discover that in the meantime the flood had found a new point of egress and was pouring through the light fitting in the ceiling. If my books hadn't now been all over the bed our mattress would have completely soaked.
As it happened I turned out to have duplicates for all but a few of the very badly water-damaged books, and those I dried on radiators and then ironed. It takes quite a long time to iron every page of a novel, but it's reasonably effective.
My editiones principes are now in a book case on the other side of the house, well away from any plumbing.
To put things into perspective, an editio princeps of the Bay Psalm Book, the first book printed in what is now the USA, has been sold for $14.2 million.
I think, however, that mine weren't worth quite as much as that, even before the flood.
Thing To Consider Today: editio princeps. This is basically Latin for first edition. The first word is based on ēditiō, which means a bringing forth.
Before you get too excited, most books only have one edition, and the valuable ones of those are all owned by people you've never even met.