[ModusOperandi]

Several reasons for each individual; but, I'd imagine it's mainly for the purpose of broadening one's exposure. The net opens up an entire world to people that would have otherwise been impossible/difficult to attain.
It also brings people with similar interests together and that, in turn, can lead to some sort of friendship.

It all depends on what the term "friend" means to you in the first place.
I've had people I can't even recall their names introduce me to others as "their friend" when I wouldn't even pit them anywhere near that.