Minecraft wiki entry for Installing_a_snapshot, associated text: Minecraft has a tendency of deleting older snapshot folders from the .minecraft\versions folder when the launcher closes. It is advised to copy the snapshot folder you have just created into a safe location.
Worlds that are not made while snapshot is activated will not be able to be played without possibly messing up the world. Make a new world when snapshot loaded. When not playing in snapshot, world that was made during snapshot will not be able to be played.