@ was used to reference people looooong before Twitter popularized it. In fact, it was users on Twitter already using the paradigm that made the engineers integrate it. Same as hashtags.
> I don't seem to remember that. Where was that used? The closest I can think is email, which is where I thought Twitter borrowed it.
IRC, chat rooms, web forums. Pretty much any sort of group social arena.
The symbol lends itself to the usage, not vice versa. That's like asking "why do people use quotation marks around quotes"; because that's its intention.
