It's useful due to convenient notation. Otherwise, you'd need to write a separate program that generates the code and call it using a separate Makefile target. (And in that case, you might check in the generated code; this is commonly done for Go packages, so that downstream dependencies don't need to run the code generator tools.)
Adding a compile-time parameter to a function is considerably more convenient than writing a code generator. Zig's compile-time performance might be a problem someday, but there is likely not enough Zig code yet for this to matter. Perhaps they'll add caching if it ever turns out to be an issue.
I think it's supposed to be cached in the same manner other things are cached, but the caching does not work great for us so far (and this may be because we are stuck on stage1 due to heavy use of async).
I have had comptime lookup table generation and similar stuff crash the compiler or take a long time to run. I fell back to external code generators in these cases.
Adding a compile-time parameter to a function is considerably more convenient than writing a code generator. Zig's compile-time performance might be a problem someday, but there is likely not enough Zig code yet for this to matter. Perhaps they'll add caching if it ever turns out to be an issue.