I think it's as Arona said -- trying to take out all the exposition to get to an ultrashort word count. And as has been said, this format is also popular with writer/illustrators. There a lot of comic-style and animation-style books out there.
I've written two stories in similar styles -- one was more of a PB with dialogue bubbles, and the other was a comic-book format. But neither went anywhere. I still think my comic book one is brilliant but my agent didn't really get it.
Usually, though, I like writing in the books I write! I have one story in two versions, a PB and a magazine short story, and I really like the short story version because there is more real writing in it, with some fun and pretty turns of phrases, although it only added about 100 words.