Quite often at the entrance to the store or mall shoppers meets a stream of air. The main reason of it is to maintain a temperature in the building to prevent infiltration and venting. A stream of air is used to maintain the store warm air in winter and cool in summer. This is especially useful if you have to keep the door open constantly. It is also a great help against the entry of insects and debris from entering the building because of air turbulence, which they can not overcome.