Also, if a relay represents a niche group or more-or-less semi-closed community and is often browsed like such, searching inside of it becomes trivial for all the moments when you want to recall something you know you saw there. This is equivalent to searching a Discourse or a Reddit.

NIP-50 can also be used in specific NIP-29 groups and even for searching multiple groups that you belong to at once. That's basically all the search functionality that Telegram, Slack and Discord offer.