1. you know when they are online (like I wouldn't know you were online if I wasn't following you)
and 2. it shows that you think that person is respectible or cool, like you.
There's a list of people online on the right side of the main forums page. I think pretty much everyone here is cool and respectable, but I don't really feel the need to follow all of them. Also, I know I'll forget someone, and I'd rather not make someone think they're less important.
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