Why snake hibernate?

In areas of the world where there is winter, snakes seek subsurface shelter during the winter months. In the mountains, large snakes sometimes share dens, presumably because there aren't enough cavities for each snake to have its own, but in other areas most do so individually. But also, by hibernating together, they cluster, thus conserving their heat, as they are 'cold-blooded'. Some snakes commonly use stump and root holes, mammal burrows, and loose soil for their dens, it's rare for snakes to winter in buildings; they seem to prefer natural underground retreats. In the Northern hemisphere, most snakes are in or close to their wintering sites from mid-October to mid-March. While in their dens they cease all activity to conserve energy (they are already cold blooded, and need to avoid freezing) and effectively survive hibernation.