The term propaganda is often used as a kind of slur. Here, I want to discuss "propaganda" used as a descriptive term. All sorts of people use propaganda, including people you likely.
Here's a definition.
Propaganda is information that is used primarily to influence an audience and further an agenda, which may not be objective and may be presenting facts selectively to encourage a particular synthesis or perception, or using loaded language to produce an emotional rather than a rational response to the information that is presented.
(Definition from Wikipedia)
The wikipedia definition above is kind of neckbeard-y in my opinion, but it is good enough to work from.
All those brainwashed sheeple!
How do you imagine someone in a cult talks? Look at how you and your in-group(s) signal agreement. Look at what you begin with as assumptions.
There is not time to bicker about philosophical this-or-that everyday, so everyone has to take some assumptions about how the world works and run with that. In some environments, it may more-or-less match up with the real world. Consider "scientific" thinking; most people most of the time just take what they are on authority, just like religious people. Those "marching for science" are not much different from people at a religious rally. They both assume they are right, and they are publically displaying their devotion to their ideological cause.
Assuming you do stuff in the real world, you most certainly operate in social circumstances that more or less force you to acknowledge certain things as true. Propaganda reinforces these axioms that create social-cohesion.
How we use propaganda
Propaganda is everywhere and it can be very useful, not just for controlling others but for controlling ourselves.
Motivation
Propaganda is super useful for doing things like reaching goals. Goals are often defined in terms propaganda taught us. Consider listening to a "work out" playlist. The music you are listening to is literally mind control waves, putting you in a kind of rhythmic trance and making you wanna go show off your enw fit body at the club or whatever. This can be incredibly motivating for many people.
Social Reinforcement
As mentioned above, propaganda is useful for signalling in-group preference. Posting such and such image can signal that you are "one of the boys" in some group. A mistake can show affiliations you were trying to hide. The more socialized you are in some group, the better you are able to recognize insiders and outsiders.
The Dark Arts
The first step to understanding the Dark Arts is to realize the darkness within yourself. Learn how propaganda operates in your life already and then you can start to help others defend against it. You may also choose to channel the dark energy for your benefit, as you come to understand it better.