Writing is a kind of communication, and the first and foremost function of a text is to convey something to its reader. In academic writing, the purpose of the text is usually to convey information of some kind (new data, suggestions for a new approach, a new interpretation, etc.) to other people within the field. Writers therefore need to take their intended readers' knowledge of the subject into consideration.
Students sometimes ask who they are writing for. A degree project in engineering or in law, written in cooperation with a company or an organisation, for instance, may have a more clearly defined readership than a degree essay within, say, history or comparative literature. For reasons of integrity (such as the importance of avoiding bias), as well as for reasons of clarity, writers need to be aware of their audience. This is something that is often discussed in essay courses, and students who feel uncertain about how to tackle the question of potential readers are advised to raise the issue with their supervisor.