Handy for three basic purposes: 1) marking the film before installing windows with unusual shapes (arches, trapezoids, etc.) that need to be cut to an approximate shape before final "lay-up"; 2) circling a contaminant (hair or fibers) before pulling the film back to remove it; 3) marking the edges of the film to check for side roll movement during the cutting process. Yellow grease mark wipes off easily.
