it is also done by a manufacturer to show that the aircraft works correctly as an airliner not just as an aeroplane. They load it up with a full load of passengers,baggage etc and then fly it on the sort of routes that that aircraft would normally fly on. a number of years ago I flew on a proving flight for the 146 (my aunt worked for Bae), we flew from Hatfield to Toulouse, got a quick tour of the Airbus facility and then back on the plane to Hatfield.
"We are the warrior race-Tough men in the toughest sport." Brian Noble, Head Coach, Great Britain Rugby League.