In Foliage Type, what the "Static Mesh Only" parameter actually means?

In Foliage type there is parameter “Static Mesh Only” and the tooltip says it will place static mesh only instead of actor. But what that actually means?
It is not enabled by default and still it gives 3 times more FPS (on high detail snowy conifer)
so there must be some possible issues in some cases. Can someone please explain :slight_smile: