Kenneth Fisher shows how to use the NUL destination to test max potential backup speed:
Then as far as SQL Server is concerned you took a backup, but the backup file is never actually created.
So why would you ever want to do that? SQL thinks you took a backup, but you have nothing to recover from. Sounds a bit, well, stupid, doesn’t it? Well, there are a few reasons.
Bonus points for Andrew Notarian’s comment.