Cover letters are a chance to fill in the "why" behind the resume, especially if the industry, type of company, or role you are applying for isn't consistent with previous experience. It answers the question: Why the choice to move from the Entertainment to Tech industry? Why go from a massive company to a start up? Why apply for a Marketing position when your previous role was in Sales? Answers to these questions reveal that one is being very deliberate about their career path. The cover letter should show that you match what the company is looking for, and the company matches what you want in an employer.
The other important "why" in the cover letter is "why this company"? It is a huge bonus in the cover letter if there is any mention of what the company does and why you are a good fit for them. These candidates are the ones who understand, at least on a basic level, what the company stands for, what they are building, why it is important, and that you are enthusiastic about it.
Below is a video giving you an overview of a quality cover letter.
The cover letter is an additional document to your resume, but make sure you do not duplicate exactly what is already on your resume. A well-written cover letter will make the employer want to read more and it will raise their interest in you as a possible candidate. Do not write a "generic" cover letter for various positions. It has to customized to each job or internship. Below are resources for writing good cover letters.
SU Writing Center (receive help in all areas of writing)