It may or may not be a vestigial remnant of the time when people had to actually write their name on all the non-verbal communications they made. They'd have to write the address of the recipient, their own address (for return or a response), place their crest or insignia, and even write their own name.
Like many other non-essential aspects of human culture and nature (tattoos for children, eating a proper meat-filled diet, dying around age 40), this trend is in a slow decline as human beings as a whole begin to reach my level of arrogance and patronizing of those whose opinions differ.
And, well, once (if) you ever go into the working world (or to school), you will encounter documents that need you to write your name twice to confirm that you were, in fact, the one who wrote it. Not that anyone normally checks the signatures unless something else seems out of place. Which is actually quite a lot like posting a message on the internet, where your name is already in place.
~Conceited young man complete with mild misanthropy, mild psychopathy, and possibly a stomach ulcer.
