I've noticed in a lot of sample games I've seen that developers use two sprite objects to represent the player and/or enemies. The "base" is generally a simple square sprite, and the "mask" is the actual character sprite with animations. They are generally pinned to each and/or put in a container together and act as one unit on the layout.
I understand why you would want this when you are using the Physics plugin (from reading the tutorial), as it allows you to have an animated sprite with a changing shape while still maintaining a simple collision box for the physics.
However, I still see this done even in games that aren't using Physics, such as the example game Demonaire in the construct recommended examples. Is there another purpose to this practice?