The author of a message is responsible for making sure the recipient understands it.
In everyday conversation, we supply information through our gestures, facial expression, and vocal tone to supplement the information we provide in words. Those supplements are not available to writers.
Writers must learn to put all their information into words.
Readers can supply writers with something words on paper can’t: a sense of writing as a means of communicating with people different from themselves.
Readers who are too much like the writer are likely to read into writing information and connections the writer should have supplied but didn’t.
Writing for an audience of others—an audience of readers who are not part of the writer’s cohort of classmates, family, and Facebook friends—helps students understand their subject better. Those others won’t be willing or able to provide the information and connections the writer assumed all readers would know.
And writing for an audience of others also teaches students the world doesn’t revolve around them—which is very useful knowledge indeed.