Cloud Wall

It counts as a safe wall, so nothing spawns against it. That is the practical reason to keep the stacks you knock loose off a Floating Island instead of dumping them: you can back an area with it and stop worrying about things appearing behind you.

There is a build trick as well. Painted white, it goes completely flat white around midday under a clear sky, giving you a blank background with no texture noise at all. It is the cheapest way to get a clean backdrop for screenshots without constructing a box out of something that visibly reads as a wall.