Over the years, I've created dozens of posters. The thing to understand about posters is they are their own art form. Posters were refined and came into their own during the first half of the 20th Century. Hundreds of posters were created by all participants in World War II to persuade people to maintain strict secrecy, cooperate with the war effort, and work hard.
Posters are often symbolic, sometimes serious, sometimes humorous. But the main thing that sets a poster apart from other media is its art value. Posters, even if minimalistic, must always be heavy on art. That's what makes posters collectible.
The purpose of a poster is to make you think.
Of course they also have commercial applications, like the one below which promotes a new allergy testing service at a doctor's office. Of course I handled the photography also. The location is the White Rock Lake bird sanctury a few blocks from my home.