Snakes do not dig holes. They don't have limbs or claws. But they colonize existing holes, often after they have taken prey such as small rodents from them. You see snakes basking in the sun on warm days and you don't see them at all on cold days. Snakes are exothermic; they do not generate their own heat like people, so they have to seek survivable temperatures from the environment. If the temperature outside is too hot or too cold, a hole in the ground is a place where the temperature is moderated, and the snake can avoid the extremes.
Snakes are carnivores, and they are not scavengers. They like live prey, so they hunt. But they are not only predators. They are prey. They are meaty and relatively slow. Birds of prey routinely eat snakes, as do mammalian carnivores like foxes, weasels, badgers, bears and various species of cat. Snakes have slow digestive systems, so they hunt infrequently. When they are not hunting, they need to hide. Holes in the ground are one of the places where they can hide.
Most snakes reproduce by laying eggs, but some species of snake bear live young. In either case, the survival of enough young to ensure the survival of the species requires that eggs or newborns are not picked off by egg-eating animals or predators. Many species of snake use dens, sometimes capacious holes in the ground, but any protected space, to lay eggs and protect them, or to bear their live young.
Snake dens are also used for hibernation. Temperate climate snakes are true hibernators because during winter they appear dead with their temperature dropping low and their hearts beating occasionally. Snakes hibernate in large groups, using the dens and their combined body mass to conserve what little heat is available. Holes in the ground are good dens, because below the frost line, the ground maintains a fairly constant temperature above freezing.