A cover letter should always go with your resume. A small number of employers will acutely regard a resume without a letter. A cover letter tells the employer exactly what type of job you wish to do and tailors your qualifications to that job.
Normally the job-seeker will see this line in a rejection letter: “We have selected the candidate who offered the best fit with our requirements.” The cover letter is the way to represent that how you perfectly fit to the company’s requirements.
During the scamming process, a cover letter may have not more then 20 seconds to get an employer’s attention. A well developed, interesting cover letter that opens a window on your personality has a much superior possibility of appealing the employer to interview you than a boring, formulaic one.