> zero improvement on end-user experience

The end-user experience is that now you can host your Debian binaries in caches and CDNs without worrying about supply chain hackers.

You can verify that file hashes match the ones on Debian's website and sleep much better at night.

If you don't trust Debian's website then you can rebuild yourself and check if Debian has been compromised.

You could already do that since Debian cryptographically signs all its package indexes, and the indexes contain the hash of all packages. The additional guarantee that reproducible builds bring is that you can re-build the packages in your own controlled environment and verify that the resulting package is bit-for-bit identical to what Debian offers.