How blower fernace door save life?

With the door on it prevents the combustion by products from getting sucked into the blower and then sent out into the house. This will kill you. Another item is combustion air. With the door off and the unit running it can suck up the combustion air and cause the furnace to run with a dirty flame.

Most if not all new gas and oil furnaces have a safety switch that will not allow the blower to run with the door off.

School is out for the day.

All is true plus the blower door safety switch will kill all power going to the unit to keep home owners from getting a hand or arm caught in the blower housing while working in the blower compartment.