To me, people collect keyboards for two main reasons, with similar rationale as how a car enthusiast might own multiple cars.

The first reason is a difference in feel and sound that people who aren’t “in the know” likely wouldn’t care for. A big part of custom keyboards is the combination of switches, keycaps, plate, mounting method, housing materials, etc. which significantly changes the sound profile and type feel. There are also many different keyboard layouts that vary in functionality, size, and ergonomics. People enjoy being able to swap between keyboards for the novelty that comes with the change in typing experience.

The second, related reason is the joy derived from building and modding. The same why a car enthusiast might enjoy swapping in nicer rims, installing a turbo/supercharger and adjusting suspension and camber, keyboard enthusiasts also just enjoy the process of assembling and trying out different combinations of keycaps, switches, and plates, adding foam to reduce resonance, and modding stabilisers for smoother keystrokes.

The last major reason would be an artistic appreciation for the piece itself. In the same way someone might buy a car primarily out of appreciation for its craftsmanship and aesthetic appeal, this is a big part of custom keyboards as well. The hobby isn’t as niche as it once was, but still small enough that many keyboards are sold as limited run “group buys”. People will pay high prices to own a keyboard with a design that speaks to them specifically.