Why demand Amen via V'Imru Amen (ואמרוּ אָמֵן) at end of kaddish versus just amen elsewhere?

There are lots of blessings where we say Amen at the end, and even lots of places within kaddish where we say Amen, so why at the end of kaddish must we command others to say Amen by saying V'imru Amen (also said in Bentching)?