I like using grids as a way of imposing discipline on my approach to drawing. There are a number of ways to use grids. In this case, I drew a grid in light pencil on a sketchbook page. I then made it a goal to fill that grid with tonal studies by the end of the day as I wandered around. The fact that each composition was a fixed rectangle helped to eliminate another decision from the process. Restrictions like this make it easier for me to focus my efforts.
There are other ways to use grids. Thumbnail sketches work really well in a grid format. Here's a "bus sketching" session:
When sketching from the imagination rather than from life, using a closely spaced grid can stimulate creative thinking. Allowing lines from one grid box to flow into the next permits an uninterrupted flow of ideas. Not all of these thumbnails will be useful, but in this approach quantity comes first. Discerning the best image becomes a process of elimination.