This is a simple rule, but it is one that many drivers don't think about. When you are driving down a city street, and you see a pedestrian crossing the roadway ahead of you, you know you must yield to them; but what about the pedestrians that you cannot see?
If you are approaching a crosswalk and there is a vehicle traveling in the same direction as you that has already stopped at the crosswalk, then you should also stop and make sure the roadway in front of that vehicle is clear before you proceed. In many places, this isn't just defensive driving, but it is also a legal requirement.
While you might see SOME pedestrians as they cross in front of that stopped car, there are others that you wouldn't see -- children are the most obvious example, or maybe a person in a wheel chair. Another person might stop to pick something up off the pavement, and then bounce back up and into your path after they do.
It is for the same reason that we do not permit passing in a school crossing area. If you pass a slow (or stopped) vehicle in a school zone, you may endanger kids that are crossing the street and hidden from your view by that vehicle. So, when driving in a school zone, we require that you maintain your relative position to other traffic that's passing through it at the same time.
Pay attention to crosswalks as you approach them, and if anyone else has stopped, slow down and stop until you figure out WHY they stopped. Then, if it is clear to go, proceed. Your caution may save someone's life.
Keep it between the fenceposts!