You use a cover letter to explain why you think you are suitable for the job, and what skills you have that are of relevance to the role you are applying for, as well as what you are hoping to gain from having such a role, and what benefits you can provide to the company. As a student you can include such things as: what you've learnt so far on your course and want to explore further in an independent and professional capacity; the value of time management and organisation; commitment to learning and endeavouring to improve yourself, etc. etc.
Basically, it's a chance to 'sell yourself', to talk about yourself and provide information that an application/CV may not specifically include.