You can
IRC with
bad breath, or in coke-bottle
glasses, or with a huge pimple and still be a
sex goddess.
Virtual chocolate doesn't cost anything, and it has 0 calories.
You can get online at 4am to say "I'm feeling down" without scaring the crap out of anyone, or making them tired before work next day.
You can flirt outrageously and not worry about the guy following you home.
If you get bored, you can leave. No questions asked. No "one for the road." No "it'll only take a second."
You can almost always find someone who has never met or heard of your SO, so you can bitch about him in perfect privacy.
And, most importantly...
There is no /ignore in real life.