Animals do this so they can survive. During the months of hibernation. There is generally a weather change, With this change there comes a lack of resources or food. These animals that hibernate would not be able to find enough food to keep them going on a daily basis.
Bears is the first animal that comes to my mind when hibernation is mentioned. My image of bears is them wandering through the wilderness plucking salmon from a river or berries from a bush. Maybe stealing some of the bee's honey. I think we all have an image of them stealing the honey.
The fish in the river and the Berries on the bushes, the honey from the bee's (who also hibernate). None of these food resources are available in the winter time to sustain the bears. They are not migratory animals and no new source of food enters the area which they habitat in.
The only way to survive the winter is to hunker down and use as little energy as possible for the coming months. A storage of food is stored up in the body of the bear to keep it alive during these months while it sleeps. The heart rate of the bear will slow down during this time. I would have to check up on how many beats per minute.
Survival. To keep alive while the food resources have dried up. Conservation of energy until the food resource returns.