When you draw on location, you have the ability to decide what to put in and what to leave out. (No need to draw every single window!)

That's one of my favorite things about sketching. You can emphasize the part of sketch that tells the story in a number of ways. Sometimes you may do that with the composition of the drawing, putting the most important element in the foreground. Or you may use a different thickness of line to make certain things stand out.

Another option is to use selective color, as I did in this sketch for last Saturday's column. I chose to leave trees, people and environment without color in order to draw attention to the graffiti.