It's used as a sarcasm mark, or to show the writer's opinion that something is strange, ridiculous etc.
Wario Forums
Just a Wario Fan
tahutoa
Okay, so yeah. The first part I didn't know about, though, I don't know if I've seen it used for sarcasm before.
Crescent-Moon Villager
5! = 5*4*3*2*1