Owing to their narrow structure, no chewing mechanism nor limbs, a snake has to swallow its prey as a whole. In fact, their success and diversity is partly due to their ability to swallow prey relatively larger than their own body size. So inevitably, they need to process the parts like teeth, bones, exoskeleton and things of such nature.
Snake saliva not only eases swallowing by lubrication, but also
contains powerful enzymes to break down tissues and even egg shells.
But snakes generally cannot digest keratin (claws, hair) or chitin
(arthropod exoskeleton). Many prey are covered with tough hide and if
a snake relied solely on its digestive juices, it would take a long to
time to get through to the nutrients. So this is speeded up by snake
venom. Venom not only immobilises prey but also starts digesting the
prey from the inside. A study showed that when a fer-de-lance
(Bothrops asper) is deprived of its venom it took 12 days to digest
a rat instead of the usual 2-3 days.
However, there is no consensus among different theories arguing the utility of a snake's venom. The above mentioned opinion is based on digestion hypothesis of venom utility.
Please refer to this excellent review and a summary of a snake's eating habits
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