Who's the audience for informational writing?

In a recent article in Edutopia, it discussed students’ motivation for writing and the importance of having an audience to write to. In the school where I teach, writing is an overall school goal that we are trying to improve. While our MEAP scores improve every year, we are still not where we should be. Students across all grade levels find writing a monumental task. According to the article, by letting students choose their audience, it gives them a connection and a purpose for writing. We all know that when students can make a real world connection, learning takes place. The question that arises for me is this: “When students are taking the MEAP and they have to compare and contrast the three states of matter,” how do teachers explain to students who their audience is and that they want to hear about this topic…prior to the test?