I'd argue reflection is very much a feature for libraries. You wouldn't use it directly, but your JSON / YAML serialize is then built on top of it. So are your bindings for scripting engines like Lua.

You can already automatically serialize/deserialize arbitrarily nested structs since C++17 (using Boost.PFR). Since C++20, you can also serialize/deserialize the struct data member names automatically.

For many useful use cases, you don't need C++26 reflection at all. E.g. https://www.linkedin.com/posts/vittorioromeo_cpp-gamedev-ref...

There are a lot of things that are very very important for a tiny niche. In any non-trivial project you will end up with a lot of custom libraries and some of them really benefit from some obscure feature that no place else in your project would want.

Also nice for UI tooling; game tools, debuggers, etc. Pull apart a struct and display it on screen and not have to patch the UI tool every time you change the struct is pretty nice.