I thought I'd put up a classic beauty. This guy, like many other males in the animal kingdom, has resorted to flashy, bright, and beautiful colors to attract the females. So the question is, why? Females put a lot of effort into reproduction; eggs are more energetically costly to make than sperm, not to mention the costs of rearing the chick after it is laid. Females typically (at least on land) mate fewer times than males, and with fewer individuals. Because of that, females are super choosy about who they do mate with, often wanting to make sure that they are getting the 'best genes' for their offspring.
Their choosiness often results in visual signals of a male's health and well being. In this case, by growing a long tail, they show they can gather enough food to grow such a tail and are still strong or fast enough to escape predators. The size of the eyespots on the tail has been linked with size of the bird as a hatchling. So picking guys with bigger eyespots may give the female bigger chicks. Bright plumage means that the bird does not have any parasites, this may mean that the male has some nice anti-parasite genes to pass on, or at the very least, that the female won't get any parasites while mating with him.
In short, this beautiful eye-candy is meant to show the females that they are quality picks.