Why do airline flights have to have flight numbers?

Each flight route undertaken by an airliner is booked in advace of take off with the air traffic control authority for the country of departure. This will be allocated a flight number and will be preceded aby the airines call sign (i.e. BMI is "Baby"). This means when a pilot alls in they know what route is booked against the flight for special reasons, to make sure tat no one should be on that route at that time, That the transponder in the aircraft is transmitting that for ATC to help to identify who the lane is and where its supposed to be going plus its designated altitude.