A time keeper in a 100 m run is advised to start the stop watch by seeing the smoke from the starter's gun and not by hearing the sound of fire. Why?

sound seems to travel very fast when we talk to each other it seems as if we hear the sound as soon as the lips of the other person moves 
in fact it does take the sound some time to travel and you  can tell this if you watch a cricket match or see a pile drive banging in a pile