At the core of the explanation of why certain groups of organisms exhibit the traits they do, is the serendipity of genetic change through time.
Venoms function primarily for prey capture.
All snakes are carnivorous, and must subdue living animals for food. Therefore, they have evolved three methods of catching and subduing their food. The snake seizes its prey, grasps it with its teeth and swallows it whole, usually alive. One method of subduing larger prey is through the use of constriction. Constrictors grasp the prey with their mouths, and throw loops of their body around the prey.