Figuring out the audience and purpose of your writing can help you figure out what diction to use as well as how to speak. For example if you were speaking to children you wouldn’t use too complex wording and might be a little informal but if you were speaking to a teacher you would use good jargon and be very formal. Same with the purpose of the writing if the purpose was to convince you would be trying to persuade the readers as opposed to when your trying to inform readers you would be giving facts rather than trying to change a point of view.