god or whatever aside, i think it has to do with temperature regulation (chickens don't sweat) and attracting of the opposite sex.
chickens combs and waddles are red or purple because of the blood that is flowing to/through them. blood flows through these areas, cools, and then circulates back to the interior parts of the body.
chickens' favorite color is red so it's also a good way to attract that special rooster or hen.
i would assume that chicken breeds with big combs and waddles come from areas of the world that are particularly warm. they evolved these larger combs to allow more surface area for the blood to cool. i don't know this, but i'm using logic here.
chickens are so neat!