Ants are very interesting creatures. They are very hygienic and to keep their colony clean they designate a place to accumulate their waste, a "city dump". This is usually away from the main part of the colony. When ants die it releases a pheromone that tells other ants it is dead. Then the natural instinct of the other ants is to carry it to the "city dump". Having an "ant graveyard" keeps the bacteria away from the colony, if they didn't carry their dead ants away the colony would be filled with nasty bacteria and diseases.