With a quick glance, another person can get a good idea about you. They'll have a general sense of what ethnicity and race you may be, how tall you are, your strength, weight, if you wear glasses, your age, your mood and attitude, and more. That first impression is done quickly and partially on the subconsious level. This same, quick assessment is made by employers when they see your resume. Your resume is, in a real sense, you. Employers, most often, scan resumes rather quickly. They are trying to size you up and determine whether or not you are the right kind of person for the position they need to fill.
The cover letter in this analogy is your clothing. The clothing people wear creates an image and an aura that helps people learn more about you. The right clothing hides parts of your body that may not be the most flattering or it may accentuate parts of which you are more proud.
The cover letter allows you an opportunity to "cover" problem areas in your resume and explains what otherwise might detract from your application. Many of our college students are non-traditional, adults coming back to school after being a homemaker for the last 10 or 20 years or adults looking for a career change. A resume doesn't allow for explaining employment gaps or career changes, but explanations would be completely relevant in a cover letter.
The cover letter allows you to explain what isn't obvious about you or what needs clarification. If you are applying for a job in which you have no direct experience but feel that your skills or experiences have prepared you for the job, the cover letter allows you to make the connections for the employer. Also, consider the applicant who has worked three different jobs at the same time for the last three years. It may be confusing to an employer how someone could do that. The cover letter allows you to explain how one was full-time, one was a weekend job and the other was every fourth night for two hours. Some people may have had three different jobs in the last year because of a string of bad luck. The cover letter allows you to discuss this and eliminate employer concerns that you may be a "job hopper."
The cover letter introduces your resume and leads the employer to the conclusion that you will be good for the position. The letter should tell the reader why you want the job and why they should grant you an interview. The letter provides the applicant with an opportunity to relate the job announcement and duties directly to you. If done correctly, it will prompt the employer to continue considering you as a candidate for the job and land you an interview.
The cover letter demonstrates your ability to communicate. All the employer has is a resume, often constructed with partial sentences, and your letter. When they read it, they can tell whether or not you know how to put together a sentence that makes sense. They can see if you know how to organize paragraphs and share complete thoughts and developed ideas. As you demonstrate this in the letter, some of your personality will leak out in your word choice and sentence structure. Employers like to see this and like that you not only list communication skills on your resume but also have demonstrated them.
Why a cover letter? Because it's essential!