Single most key is layering concept. Background pages can be treated as layers, conceptually.
So, when a drawing page enables a background page, the contents become visible, "layered" behind contents of the drawing page. Let a background page enable 2nd backgroun, then, the 2nd is "layered" behind the 1st. Thus, the drawing page sees both. It's equivalent to stacked layers on clear glass. Not unlike the process Disney developed for his animations.
Hope that helps simplify the concept of sing multiple background pages. It is not necessary to use multiple background pages, but, the capability exists. Might be useful.