Apart from basic writing competence, understanding and targeting audience is the most important skill a professional or academic writer can have.

And it's one of the areas most overlooked by students.

If you're writing about a novel, do you assume your reader has already read it? If they have read it, you can skip all the basic summary.

 If you're writing a cover letter to apply for a job, what unusual part of your life experience or skills should you stress? If the audience is a multi-national corporation, with offices in 100 countries, you might want to stress your semester in Brazil. If the company has a narrower market, and works with customers in the American upper Midwest, leading with Brazil probably won't help.