Though specific practices vary widely from culture
to culture and region to region, burial is important for preventing the dead from returning to plague the living. It is a ritual which allows the deceased to rest and provide closure for the mourning.
Unburied dead – abandoned, lost or forgotten – are most likely to come back as walkers or ghosts. A common example in this age of war are corpses left on battle fields rising as ghouls.
However, burial is not only for the departed. The intense emotional states of the bereaved, be it mourning, longing or guilt, can cause the revenant to return and drag them into the realm of death if there is not a proper ritual of closure.
It should be obvious that the ritual of burial only works on the dead, though interring bound monsters into the earth is also a solid way to keep them from causing trouble for a good while.