For terrain that players can't walk through. Sure, drawings on the DL layer can inhibit movement, but they also inhibit sight. Think of the possibilities: On the Map layer, it could represent a physical border that cannot be crossed. On the token layer, it could be a heavy obstacle that must be moved somehow, or an example of terrain bordering on rough/blocking depending on your system. On the GM layer, it could represent invisible forcefields or "stop walking" points on the overworld when your players are free-moving through safe territory so that the GM can prompt a scene change or the beginning of combat.