Is the purpose of life to only work on "useful" things? I ask that genuinely.
The first point is more-or-less: the OP should be using their talents for something "better". I don't think this is a really interesting question.
Them more interesting point is: "is this project useful?"
The end product likely won't directly serve some "useful" purpose, like making money or even being played by others.
On the other hand, building such a thing is _likely_ going to teach the poster a large number of useful skills that will serve them for "useful" projects.
This is fun for him. And it is intellectually challenging difficult work. It improves his problem solving and technical abilities but obviously he is enjoying it as a hobby.