The Buddha says that all composite things are impermanent. They are liable to disintegrate or are perishable in nature. The realization of impermanence is extremely important in all level of spiritual development including Bodhisattva levels. Instruction on impermanence is imperative as a remedy to eliminate attachment to sensuous experience of this life which is the essential condition for the attainment of Buddhahood.
In most of the Dharmapala imagery the bone ornaments are the symbols of the impermanent nature of phenomena. The wrathful appearances of Yidam or Dharmapala deities are also the embodiments of impermanence. When we realize the impermanent nature of our body and phenomena we are terrified. Then slowly we understand that we all have to die, leaving behind everything except virtuous and nonvirtuous deeds done in our life time.