Paragraphs in the essay should always be written with complete sentences. When you have no choice but to outline your answer, because, for example, you do not have enough time, you don't use the complete sentence, and thus you show your teacher that you have information about it. Beside, it is very important to outline your answer, which is a sort of list of the main characteristics of your answer. So you emphasize your point in the answer, a sense of flow is achieved.
