It is essential that you edit and proofread your writing to ensure effective communication. This means that you make sure that your writing has relevant content, logical organisation and clear expression as well as fulfills its purpose(s). Editing and proofreading applies to coursework assignments and thesis writing as well as other academic writing like journal articles and presentations.
While you may get feedback from teaching staff in the form of edits, the person with the primary responsibility for editing and proofreading your writing is you. Editing and proofreading your own writing is also essential to your academic integrity: you are responsible for ensuring you have correctly cited the resources you have used in writing.