After setting out some lines and marks setting up the drawing I go over it with an eraser (standard issue Pink Pearl). I think of the eraser in the same way that I think of a drawing instrument. I think of the mark that it makes, the texture that it leaves. This comes from my early days as a printmaker, primarily from woodcuts, where you can change the field from black marks on white to white marks on black. For example if you make a drawing on a wood block to print, you can make a choice either to cut out the drawing itsself or to cut out everything around the drawing. This made a big impression on my artistic development and had a big impact on how I see relationships in a work of art.
To come back to the drawing above- erasing some of the marks also helps to set up some of the planes of the picture. It helps make the transition from lines to planes.