Up-voting @skipmiller, I offer this addendum
if it is dangerous what makes it worth it?
Emergency Landing After Takeoff
Large aircraft typcially cannot land as heavily as they can take off. It's a structural issue. So fuel dumping would be done if an emergency did not allow the time to burn down to allowable landing weight.
Maintain flight with engine/power loss
If available power did not allow sustained flight one might dump fuel. Worse if you were over mountains and needed to stay high.
There was a case of a C-130 - a high wing plane - ditching in the Atlantic. It floated for a couple days with empty gas tanks and had to be shelled to force it to sink.
If a wing tank fuel leak was causing aircraft balance problems one might dump from the opposite wing to keep the aircraft controllable.
Life Imitating Fiction
15 minutes after takeoff we lost all thrust on 3 engines. Our gross weight forced us into a controlled descent. Dumping most of our fuel might very well have kept us airborne on 1 engine, if not I think empty tanks would have limited possible fuel fire in the controlled crash. I was about to dump 45,000 lbs of JP-4 all over west Texas when we solved our engine problem.