Many people just need that extra push to buy war bonds, or maybe they didn't know that having your own garden while rations are low and need to be sent to the military is a good idea. I agree that a lot of propaganda in the past has been very negative and attacking of the other side, but not all of it is like that. There is moral propaganda and there is immoral propaganda. Obviously immoral propaganda is bad, but not all propaganda has to be that way.
Propaganda should not be used to support any war. If people want to support war, they should be able to make that decision on their own and not be pressured by any war propaganda. If a war is just, people will support it no matter what, regardless of any propaganda.
The entire purpose of propaganda is to try and shape the minds of citizens to believe whatever the state wants them to believe, whether it be that the state's cause is just or that their enemies are inhuman, it always serves as just glorified lying to citizens. Citizens should have a right to choose whether to support their country or not. People shouldn't be forced to participate in organized murder just because they just happened to be born in a certain geographical area.