Hibernating animals stay at a low body temperature throughout the winter so that they will not need to use so many body resources.  If they maintained a high body temperature during this time, they would need to expend many calories to do so.  During the winter, food is much less available for these animals and they would likely be unable to find enough food to maintain that body heat.  They would starve.

Induced hypothermia is used mainly on people who have suffered cardiac arrest or spinal cord injury.  The reasons for the effect are not completely clear, but inducing hypothermia does help prevent such patients from suffering strokes.