Unless you are taking a course in creative writing, essays are generally meant to help you consolidate what you have been studying. You are not asked to answer the question in the title ‘off the top of your head’ nor on the basis of some prior knowledge. You are expected to take the essay as an opportunity to scan back over what you have been reading or doing and select relevant material from that. The tutor who marks your essay will already have in mind a range of course material that could be used in answer to the question. Your ability to make good, relevant selections from that material is part of what she or he assesses.
If you miss out some of the important ideas and information, or make the occasional strange selection, it may not matter provided you make good use of other relevant material. But if you try to answer the question entirely from knowledge drawn from outside the course, you may run into trouble. In effect, what you are doing is focusing attention on the fact that you haven't used the relevant material in the course and the new knowledge available to you. Also, it is often very difficult for tutors to evaluate your account of material they are not familiar with. They are unlikely to have time to read, view or listen to your sources. So, you may find you receive a higher grade for an essay that is based on course material, even if it has been put together hastily, than for one on a topic you think you know much more about.
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