Back in the dark ages of marketing, the 1960’s, a Canadian professor Marshall McLuhan coined the expression “The Medium is the Message.” For whatever reason that caught on and he became famous for it.
There is more than a grain of truth to it – though it is grossly exaggerated.
In this age of cynicism, twice-burnt consumers, over-hype and under-deliver, and a gazillion marketing messages a day, believability becomes huge.
It used to be you could make a statement about your product or service, back it up with pictures, testimonials, etc., and you were off and running.
There are still places in the world where advertising like that works.
The U.S. isn’t one of them.
You always have to ask, what does it take for what I’m saying to be believable, and believed?
An important part of that is the medium. We see this every day and in many ways:

Joe is supposedly a successful businessman but drives a beat up 10-year-old car.
Fred sends out an email about how he can make you a millionaire. The email comes from a gmail account (free email).
A marketing company is promoting their expertise with postcards that were built using clip art.

Such elements as the kind of paper your message is printed on, the marketing channel used, all come into play.
The moral of the story is put yourself in the shoes of the person you’re trying to reach, and ask, what would it take for me to believe you? What would make me disbelieve you.
It’s a valuable exercise.