You have to think of shadows quite literally, you use polygons to define the outline and they are rendered as an offset.
This makes it easy make make simple shadows but it is also completely useless if your trying to make a shadow of something that is taller or shorter than a default wall.
Also layers aren't just about the value, they're rendered in the vertical order they are on screen based on their origin. Setting a higher layer helps when you have objects on the same level like a painting, by default it would fight a wall for priority but setting the layer higher means it is always rendered later and on top of a wall.
