I build a lot of "useless" stuff too. Where by useless I mean useful to me, but isn't ever going to have customers.
A. Educational value is a thing. Not everything needs to be a business with customers. Not every project needs to make money. Many of my projects teach me something about science, tech, or arts.
B. Fun is a thing. Building stuff is a way to enjoy life for me, similar to spending time on the beach.
A couple of my more "useless" projects:
https://dheera.net/projects/4x5/
https://dheera.net/projects/mnist-clock/