The key signature notates the key. That’s what key signatures are and that’s what they do. They tell you things like “all E’s herein are E flats unless otherwise notated”. Pretty important if you’re sitting down and reading a piece of music you’ve never heard before.Exactly - and that also doesn’t let you know what key the piece of music is in.
I’ve said these are not laws, they’re tools of communication. If you don’t find key signatures useful, that’s fine. Maybe you don’t read notated music, which is totally fine, many very talented people do not. But if you’re going to argue that they’re not useful, which seems like what you are doing, then you should expect resistance from people who find them to be a pretty useful tool, as they’ve been pretty useful for a very long time, and you should probably be presenting an alternative that is more useful, which I’ve not yet seen you or anyone else do.