There's another reason.

Propaganda that is obviously lies is saying to people "look everyone is full of shit, we can make it too, see? The West are also full of it, you just aren't used to their crap, but it's still crap. What you can rely on is that you're one of us and you should follow what our leaders say, because you at least know the culture you grew up in".

Basically you create noise so that nobody can trust their own intelligence when deciding what to believe. Either they don't have the information or it takes too much work to get, so they are forced to fall back on some sort of group decision that's presented to them by above.