> in general sha-256 is already really fast. Why bother with something faster?

Have you never sat there waiting for a hash to finish computing? I regularly do and I'm not even dealing with large data. It's very slow, and worse than that it's both very slow and also entirely sequential! (So is BLAKE2 - BLAKE3 is parallel.)

Worse than that - it's an astronomical waste of energy if we could be doing the same thing in less time. I wonder how many hashes humanity runs a day?