in ammonotelic animals kidneys do not play any role in the removal of ammonia so what is the use of these kidneys?

Kidneys are not resent in ammonotelic organisms. For example, excretion in Amoeba takes place mainly by the process of diffusion. In platyhelminthes, flame cells are organs of excretion. In arthropods, Malpighian tubules are organs of excretion.