The purpose of the cover letter is to introduce yourself to an organization, demonstrate your interest in the company or a specific vacancy, draw attention to your resume and motivate the reader to interview you. Often this letter is the first contact you have with a prospective employer.
It is important to highlight your skills, knowledge and suitability for the specific job you are applying for, and an opportunity to make the recruiter feel comfortable that interviewing you won't be a waste of their time, and that you would be a suitable candidate to put forward. Get it right and you will create a very strong first impression with the employer, remember you only get one chance at a first impression. A cover letter can often be the deciding factor in the choice of who will be interviewed and who will not.