The United States was first to use film as propaganda, though most of us are probably unfamiliar with the Capra film series that began with Why We Fight and included titles like The Nazis Strike. Japan followed America's lead on using propaganda films.
While propaganda films were needed in America mostly--but not solely--because prior to Pearl Harbor, the US held a non-interventionist foreign policy, one reason Japan used film propaganda was to encourage their population to support their expansionist vision, a vision ironically taken from American expansionism--that encompassed Hawaii--prior to World War II.
Propaganda was used by Japan (and America) to help instill a feeling of hatred for what the enemy was doing. One means of doing this in Japanese propaganda films was to show the merciless hardships of war in order to develop empathy for their soldiers' plight and the reverse feelings for the enemy's aggression. Anthropologist Ruth Benedict is quoted as saying that Japanese films had a "propaganda courage" not apparent in American films because of the tragic war detail Japanese propaganda films included.