It's very easy to assume that simply by producing a piece of writing you have succeeded in communicating. Everything seems perfectly clear when you read it over, so why wouldn't somebody else understand it?
The short answer is that, if you haven't put considerable effort into providing cues, you are expecting your reader to be a mind-reader. You have spent hours preparing and writing your essay, and as a result have formed a very detailed mental picture of the topic, which you automatically relate to the words you have put down on paper. But the reader can't see this picture inside your head; they can only form their picture of the topic through a process of creative reading. Your job as a writer is to make it possible for your readers to reconstruct an adequate version of your mental picture, or approach.
This aspect of writing is very different from simply conveying facts, or pieces of information. You need to find a way to communicate the context, or mental picture, which makes those facts seem significant to you. In other words, you need to answer the so what? question.
For a university essay, the set question represents an area where your mental picture will overlap with the reader's previous awareness of the topic. Making sure your essay relates clearly to the question is an easy way of making sure you are communicating clearly and effectively. You also need to bear in mind the position which you occupy in relation to the question's academic field.