Question

Why do you need to understand the purposes, audience, and context for a message to know whether a specific set of words will work?

Answer 1

Because the way people responds is really different depending on the way we convey a certain information, which make it important to  understand the purposes, audience, and context for a message before we even made the script.
For example, if the intended audience is children in their elementary school, we need to make the words and intonation seems friendly and interractive. If the intended audiences are group of professionals, it would be better if the words and intonation made into serious and to the point

Answer 2

When you are giving a speech, you should understand the purposes, audiences and the context for a message so that you can make your speech relevant to the size of the audience, make the speech informative to them, and relevant to them.