Writing ideas down preserves them so that you can reflect upon them later. Writing out your ideas permits you to evaluate the adequacy of your argument. Writing stimulates you to extend a line of thought beyond your first impressions or gut responses. Writing helps you understand how truth is established in a given discipline. Writing equips you with the communication and thinking skills you need to participate effectively in democracy.
Writing is an essential job skill.
Be sure you take time to proofread carefully. All professors want clear, carefully thought out, error-free papers. Be sure you understand the purpose of each assignment and the standards by which it will be judged. Others will require your reflections on personal experience or specific texts. Some professors are looking for analysis while others expect a carefully argued interpretation.