Why does a peacock spread its feathers?

A peacock, Pavo Cristatus, has multicolored feathers so he can attract and mate with as many peahens as possible.

The bright plumage draws attention to the male; showing off his vibrant feathers is a sign of health. His unnaturally large and disproportionate tail, which would otherwise hinder an animal's survival, is proof of his successful survivability; this is known as Handicap Theory. The loud peafowl call that echoes through the woods is a symbol of his dominance and vigor. His heavy tail-waiving and dancing is a sign of strength.

The entire display has the peahens swooning, or scientifically speaking 'sexually selecting' the male, which in turn ensures the production of future offspring.