Reading the previous greenberg commentary gave me a little bit of insight on the "why" of collage. These artists were pushing the limit with cubism, and stretching the boundaries of what was being done. Their reasons may not have been clear, but it is interesting how the layering of the substrate came about.
I am thinking about layering in my art specifically. I don't layer things for reality, for 3d punch...I tend to use the various layers and pieces of ephemera as paints. Each texture becomes it's own paint or color...and therefore can fill the area. I have not really exploited the 3d possibility of adding something to the base of the art. I have seen collage work that has 3d pieces built into it, using foamboard and repetitive images, but not until reading this article did I feel as tho the layers in my work could become something else besides a type of "paint".