Originally it came from highlighting how people did things unprompted, like "nobody asked her to do this but J. K Rowling said that wizards used to shit their pants and magic it away"

But now it's turned into nothing more than a signifier that this is meme, like using "pov:" when the accompanying picture is of the subject

I think it started to contrast with a really ridiculous action in the punchline image ("nobody asked you to do this"), but then it just got put on everything.