It's the first thing a prospective employer sees. And if it's not done correctly, it may be the last.
Cover letters are one of the most overlooked aspects of the process of impressing an employer.
Even before a prospective hirer sees your resume, they will probably take a glance at your cover letter. And if they don't like what they see, they may decide to cut you out of the running then and there. It has to be eye-catching, but it also needs to be very, very professional. There's a fine line. Unlike some other presentations you may make later in your career, when you are looking for a job, simplicity counts.