The human body is a tricky form to design for. One, we are all very different. Two, our bodies move constantly and thus a garment needs to function properly as well as look great. Testing ideas on the dress form addresses the 3-dimensional aspect of our bodies. I can see how a piece of fabric behaves on the body--how it drapes, where it makes sense to add volume, and how to adjust the silhouette.
See below for an example of my work. I cut out two large pieces of muslin (a cheap fabric great for testing of design ideas). Then, I got to work! The sample on the left is a skirt or a bottom to a dress. The sample on the right is a top (as a separate piece or with a skirt added to create a dress). The sketches show you how I interpreted what I saw on the dress form into my own designs.
It is great to have a variety of ways to generate new ideas. And while I tend to prefer photography, I realize how important draping is. So, I will make sure to incorporate it into my work more. The options are limitless!