Typing your name

I just received an email from a friend. It was a pretty ordinary email. At the bottom, instead of writing her name, she just put her first initial, J (initials have been changed to protect the innocent).

I guess I don't really understand the point. If you are going to write something to signify that it is you, then why not just type your name, I mean, we aren't talking about some huge expanse of time being taken up by typing out a six letter name. At the same time, if you don't have to time to type the full name, why not just leave it out. After all, the sender's name is right there at the top of the screen so I already know who it is from. It is just redundant at the bottom.