As far as I can remember, it's only used to set up a site Profile. I prefer not to have one of those, but I know many folks have them. I can't think of a reason to be able to search site wide for them, other than curiosity. Unless someone is in a group that I belong to, I don't see why I'd be interested in any particular person.
There's an item on the Trello list to be able to search messages by sender, but that's the closest I could come on finding anything related.
Duane
Duane