One of the most important things to think about is how are people going to interact with your poster. These people are called "users" and you need to think about what experience your poster will evoke with them.
Your poster should have a focus. We call this a target. You are trying to grab a specific audience so you aren't wasting your time, money and energy. Marketing for products are often carefully designed to meet the needs based upon the demographics.
Creating work for specific groups works, and evidence is the billions spent each day on getting people to purchase the next thing. What works for toddlers will not necessarily work for teenagers, or the elderly. So it is critical that there is a design awareness for what the product is for and whom it is intended to use it.
Culture isn't only based upon factors such as religious backgrounds or different nationalities. Culture could also be an example of what is trending and what isn't. For example words used just three years ago may not be used today. It is important to have the design awareness to know what to use so you don't offend your audience, or turn them off by being lame.
Below are examples of posters for very different demographics. Therefore the intended UX is varied.