If you have not had any previous contact with the employer, cover letter is a must. If you are targeting a newspaper ad, or contacting someone whom you have been advised to get in touch with by your friend, aunt or whoever, you should definitely include a cover letter explaining your situation. If you have arranged an interview with the potential employer over the telephone and are sending your resume to him, you should also include a cover letter. And, even if you are sending your resume to an old friend of yours, you should attach a cover letter explaining why are you looking for a job and why are you the best person to hire. It takes time to go through a resume.
Employers decide from the cover letter whether the resume is worth that time.
Most employers only scan through the resume and then read the cover letter, so your cover letter has to be as well prepared as your resume. After all, your resume does not say why are you looking for a new job and why are you interested in the position you are targeting, so the cover letter has to answer these questions. Your cover letter is your opportunity to stand out, to tell the reader what to think about you. The cover letter is your way to distinguish what you have to offer to the employer so that the likelihood of an interview is at least doubled. Send your resume and cover letter to the person who can make the hiring decision.