How?

I do not get.

Two Royal Air Force (RAF) Typhoon jets were scrambled to assist an American Airlines passenger plane shortly before it landed at London's Heathrow airport on Tuesday after a woman attempted to gain access to the flight deck.

How exactly do air force jets "assist" a commercial plane? Or by assist do they mean "hang around to see if they have to shoot the commercial plane down in case the cockpit is breached by a crazy" type of assist? Got to wonder how much it cost to scramble those jets, and if having an air marshal on board would have been the better -- and more economical -- option.