i have used it all of twice once was when i ran into a post about something that sounded familiar and sent a dm to the poster turned out it was a friend of mine's account, the other i messaged a moderator when a bot for the sub quit posting updates and they fixed it.
They also do things like the chat, live threads and other nonsense that I will never use but clearly will take a decent number of man hours to build.
There are the user avatars, the different events like Place. Or having to implement their own image/video sharing back-end.
Does that add up to $450M/year of R&D costs? I don't know.