They use it for prey capture. Contrast the behaviour of the Asian 'Elaphe-types' with that of the Americans (which have recently undergone a secondary loss of the gland). The Asians are actually pretty pathetic contrictors but absolutely bite the living crap out of the prey item (quite viciously). The Americans do little biting (other than the intial strike) but are robust constrictors.

The evolutionary key here is that there was no evolutionary selection pressure for the development of fangs unless there was already a potent venom to deliver. Sort of like all primate lineages being able to make noises but we are the only lineage with the highly developed piece of architecture called the voice box.

Venom in the advanced snakes are analagous. The heavy bodied primitive snakes dumped the muscle that was essential for prey capture in order to become more gracile and agile. They needed a new weapon. Enter venom.

Cheers
BGF

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