It's basically a form of shock/surprise humor in most cases; the cropped out bottom text says something absolutely unhinged, or expected, and to read it, you'll have to try hard
That way, text hits unexpectedly because you can't "pre-read" the cropped text and get a gist while you start reading the whole thing, creating a suspense when you do read it after you've read the rest
It's basically a form of shock/surprise humor in most cases; the cropped out bottom text says something absolutely unhinged, or expected, and to read it, you'll have to try hard
That way, text hits unexpectedly because you can't "pre-read" the cropped text and get a gist while you start reading the whole thing, creating a suspense when you do read it after you've read the rest