If there was a map in the Mat Editor that allowed us to select which decal to use as the projection, we could position maps, but more specifically masks onto scene objects by using decals. This would open up the possibility of easily animatable, layered materials that feature moveable decals within the layer stack. Or a new level of scene interactivity for complex procedural layered materials.
Another use case of this could be for applying displacement via decals but as geometry displacement with the displacement modifier.