Thumbnails like anything else has a purpose. In the art world that purpose closely resembles the purpose of an outline in the world of literature. Both are used as a guide or experimentation to manipulate your work to see which pieces are most effective and how the location of the pieces affect your work overall. Thumbnails give you an idea how shapes combined with the space or plane around them give different effects and as you play around with them it can lead to other ideas or concepts to help enhance what you design. These advantages and lessons makes thumbnail sketches a crucial step in the developing stages of any art work.