The impact of knowing your audience/ readers is huge! Imagine you’re writing a letter to your grandmother to tell her about your favorite video game. What details might you include? What might you leave out? How crazy will she think you are? Will she call the police?
Now imagine that you’re writing on the same topic but your audience is your best friend. Unless you have an extremely cool grandma with an Xbox, it’s likely that your two letters would look quite different in terms of content, structure, and even tone.