I've seen it used usually in those ms-paint anime images. It might be because it makes the person look retarded. It might also be an attempt at being cute. It is the Tumblr crowd who regularly type in that manner after all.
im being called out. i do it because im a legit retard with mild dyslexia and terrible typing skills and im too lazy to proof read before hitting post reply.
Any time I fuck up spelling I try to edit it within the narrow window that says it's an edited post. I also do it when I use the same word in sentences but that's a whole other thing that makes less sense.