Why do some plants capture insects?

Insectivorous plants - some plants capture insects and other tiny animals and use them as food. They do not devour their prey by chewing but decompose them in a mixture of enzymes. The pitcher plant attracts an insect to its large showy leaf by means of sweet smelling nectar. The leaf has a treacherous lip which precipitates the unwary victim into a deep hollow pitcher full of a digestive 'broth' which soon decomposes its body. Other plants, like the Venus's flytrap, snap their leaves shut on their prey as it prowls about the trigger hairs glistening with drops of nectar. The sundews secrete a sticky fluid.