Any reason you won’t just use a hash?

Yep. Almost every company uses multiple vendors for things. Suppose you use a tech support helpdesk and you don't want to waste time dealing with banned ex-customers. You can't import that list of hashes into Zendesk or whatever and tell them to blocklist them.

Substitute "billing company" or "authentication provider" or "fraud detector" for "helpdesk". There are times when it's not sufficient to say "don't do business with SHA-256 hash ef61a579c907bbed674c0dbcbcf7f7af8f851538eef7b8e58c5bee0b8cfdac4a". You need to say "John Smith is banned".