I see a few people doing this. Your user name is on the left, so why is it necessary to sign your name at the bottom of your post too? 
Another thing is with email. You know who it's from, it's in the 'From' field in the email. So then why do you need to sign it?
Another thing is with email. You know who it's from, it's in the 'From' field in the email. So then why do you need to sign it?