The toucan's bill is useful as a feeding tool. The birds use them to reach fruit on branches that are too small to support their weight, and also to skin their pickings. In addition to fruit, Toco toucans eat insects and, sometimes, young birds, eggs, or lizards.
As a weapon, the bill is a bit more show than substance. It is a honeycomb of bone that actually contains a lot of air. While its size may deter predators, it is of little use in combating them.
Toucans are considered frugivorous, meaning they eat mostly fruits and seeds. Although toucans will also eat small insects and small reptiles as well. Maybe that's why they have such large beaks: to help them eat a wide variety of foods.
