Some forum software also hyperlinks these “@ mentions” of usernames automatically, and notifies the person that they have been mentioned.
What did the @ sign add that the usual written comma and space after an noun of address didn’t?
In the days of IRC, there were often channels with tons of traffic and the scroll would be going by pretty fast. Not a problem if the conversation is between two people as you pretty much just ignore what other people are saying. People would use the @ symbol in front of your name to signify that they were jumping in. Not strictly necessary, as you say, but it helped, especially on a crowded channel and when you’re throwing out comments from the peanut gallery.
Happened to catch this article recently and remembered this thread. So, for further reading on the @ symbol:
:dubious: Sounds like that was *exactly *what you needed an explanation for.
Eh. . . no. . . The actual question in the GQ was
And in the various responses, I think we have as close to the reasons as we’re likely to get.
My response that you quoted was itself a response to a) an answer to a question that I was not asking, and b) something that in this day and age, I kinda think most people are hip to.
Sarcasm often translates poorly in a printed/typed medium.
But thanks for bringing this back up. [No sarcasm intended!] 
English didn’t used to have the “towards” sense for “@” either, though. English prepositions are fairly ideosyncratic, and while “@” meant “at” in the sense of “6 cwt. cement @ $2/cwt. = $12”, it didn’t use to mean “at” in the directional sense. Nobody would have addressed a note to “Oscar Wilde @ the Albemarle Club”, for instance. That was a computer-driven innovation in English as much as in every other language.