I dont see the use of this but ok,why would i use that.
In sites i dont want them to se my mail i hide it.
The people i wanna talk i give them my mail.
The rest of the people i dont like i ingore or block.
They may be good for something. I still don't know for what , but they are not good for Facebook.
Like imagine you have to write 50 letter long email before password...