Private relays for businesses. Generally you trust the corporate network but don’t want to be held liable when things leak. It’s the same idea as DKIM on email, sometimes that bites you in the ass when it leaks, but if there is no signature you can claim it was forged. 

 I can imagine that business would not be happy about notes leaking even without a signature. If it would all be encrypted on top then it might be a nice solution for them.