To add to what Nightstalkers said there, the original hacker geek community invented this stuff out of paranoia. They feared government tracking with ancient bots, so they typed in stuff the archaic bots wouldn't know to detect. Somebody asking for "pr0n" for example, thought he was getting around the bots who were sniffing for porn traffickers. Of course, like all stupid things, this language of paranoia became a status symbol and spread with so many hacker wannabees out there.
During the same time, people too lazy or hasty to type things out developed crap like emoticons, and shorthand such as "lol" and "afk."
Both means led to the end of sheer laziness. Which what I think everyone hates.
Now, it lingers not for paranoia or coolness, just for simplicity. I can understand the paranoid or the posers. However, I cannot understand sombody so lazy they type this:
"ne1 hav 2 look 4 me i b at skul 2day"
Yeah, sucker...go learn some English while you're there.