One way hashing came up in the context of sharing email opt-outs without sharing the actual email addresses. (Send a list of hashes of email addresses and the hash function and require that no one whose email hashes to a value on that list may be emailed marketing material.)

It's been many years now, but we did some canonicalization of email addresses before hashing [downcasing, stripping whitespace, stripping periods (gmail), stripping any local tags (plus-sign), and maybe a few others I'm forgetting] since false positives were "safe".