Let's take a look. I have just passed Officer Brown who is sitting on his bicycle by the curb and talking to a friend. On my right side, or passenger side, Officer Blue is about to cross the street.
With RJ at the wheel, I move along a lightly traveled street. Officer Blue steps off a curb and unknowingly steps into a blind spot. My driver, RJ, cannot see because of the windshield side post and the right side mirror. The blind spot will only last for a fraction of a second in a moving vehicle, but that is long enough for a driver or pedestrian (maybe you) to have a serious problem.