If a plane is having an emergency and needs to land sooner than planned, the pilot may have to dump fuel to lighten the aircraft for a safe landing. In the event of a crash landing, the risk of serious fire is greatly reduced if the fuel tanks are almost empty.
This video shows fuel being dumped from the jettison system in the left wing of an American Airlines Boeing 777-200. In this case it was a medical emergency.
and this KLM flight KL888 from Hongkong to Amsterdam on 25th Feb 2007 had to return to Hongkong because of a malfunctioning Krueger flap. 80,000kg of fuel was jettisoned!
“The 747-400 has a fuel jettison system to allow the airplane’s weight to be reduced quickly so that a safe landing can be made after an inflight emergency.” Boeing Airliner magazine
on January 1 2009, Air France flight 332 outbound from Paris to Boston u-turned and made its way back to Paris. Dumped fuel all because an unclaimed cellphone sparked fears of a bomb on board.