That provides multiple advantages. Unlike GitHub’s unreliable automatically generated files, a fixed file can be hashed or cryptographically signed by the project (with SSH signatures, Signify, PGP, etc.), and later verified without having to extract the files first or check out the underlying repo.
Another thing many projects aren’t aware of: if your project uses Git submodules, anyone using GitHub’s autogenerated tarballs will be unable to build your software, because those don’t contain submodules.