Heh, first post of mine here in a while. Anyways...
This flag works similarly to the No Enemies Flag, only enemies can walk here. They just can't spawn on this combo. A good way to use it is put it on every tile of a screen except one. That way, all enemies would spawn here. Basically, using this flag like that would create a little "Enemy Spawner" that'd spawn every enemy at the same time. Or they could be used to keep Leevers from spawning on certain tiles! Combine it with the No Enemies flag, and you'd have the perfect way to block Leevers from getting on certain spots... I mean, even with the No Enemies flag, isn't there a really small chance that the Leevers would SPAWN on the tiles you don't want it to get on, and thus get stuck there?