What does a petal in the plant do?

Petals are mainly for show, reflecting bright colors like runway lights for insects to pick up/deposit pollen (and flag where they can feed on tasty nectar). Some petals have been adapted for different purposes, too. For example, the flowers of a monkey orchid have evolved to look like monkey faces to scare away anything that might trample them. Other than show, though, they're not used for much.