how can understanding a documents intended audience help you determine in its purpose ?

If the audience is very young, the primary goal is to entertain. For older people, however, there are a variety of different things the writer could be trying to accomplish, such as persuading, entertaining, and even informing. For that, you'd have to look at the content.
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HermioneGranger919 HermioneGranger919 · Answer 2 · 2017-09-18T20:48:47+02:00

A document is usually intended for one or two groups of people. If you look at the type of text a document uses, you will most likely find the type of audience. For example: If you come across a document that has the following sentences: "Hey! Have you guys ever gone to the park? It is super messy! We need to clean it up!" You know that they are probably addressing children. But if the sentences are: "Good evening. I have brought you here today to discuss our trashed park. We need to do something before it is to late." Then you know that the document is probably addressing adults.