Clearly defined goals for the essay will make planning it much easier. A goal in this sense is not a personal ambition (such as "I want this essay to get a mark of at least 65%") but an orientation for your writing.
To avoid producing an essay with no clear orientation (either because it is mostly a list of unrelated points, or full of phrases from the question which are just repeated rather than discussed), you need to distinguish between things you want to say (ie content), and things the essay is required to do (ie showing structure and providing cues for the reader). Considering the essay's rhetorical situation and the expectations behind the question will help you make this distinction.
The goals you set should be operationalized by identifying the concrete strategies you would use to achieve them - students often set goals for essays which would be hard to achieve in a 300-page book. Discussing your goals with your tutor, or other students, can often be a good way of identifying goals which are over-ambitious.
Explicitly stating the essay's goals (eg in the introduction) is a way of taking the needs of your reader into account, as will make it much easier for them to understand what the essay is attempting to do.