A cover letter can be a valuable marketing tool for you if it's well written and uses the right approach with the right information.
As a hiring manger, I estimate that at least 75% of the cover letters I've read were pretty much a waste of time to include with the resume; either they are poorly written or they do nothing to enhance the resume. Here are the top two cover letter blunders that I've seen over the years. Make sure you avoid them in your cover letter.
Avoid using a "boilerplate" cover letter.
Too often, I see cover letters that are obviously written in a generic manner and used for all applications, regardless of the employer or the job. They usually simply recap the applicant's experience and skills straight from the resume, without regard to how they match the job for which they are applying.
You might as well not bother to include a cover letter like this.
Cover letters only add value if they point out to the hiring manager reading them exactly how your skills and experience match the specific job that is open (just as your resume should do). The cover letter should gather the aspects of your resume that most closely match the job requirements and present them in a way that convinces the hiring manager to pay attention to your resume.
The best cover letter I ever read -- and I've read hundreds over the years as a hiring manager -- did this in a manner that was both concise and elegant. What this applicant did was simply list the major requirements of the job, straight out of the job posting, and under each requirement list the skills and experience that matched. It was all done on one page, and it made it immediately apparent to anyone reading the cover letter that this applicant was a perfect match for the job. I couldn't wait to read the resume and, in fact, hired this person the following week.
That's the power of a good cover letter, so don't give yourself an immediate strike by using a generic, boilerplate cover letter.
Do not try to include everything in your resume.
The purpose of your cover letter is to get your resume read, not replace it.
I frequently see long, rambling cover letters that attempt to repeat everything in the resume, only in a narrative manner. If your cover letter includes everything in you resume, why should the hiring manager read you resume? And if I do and the resume simply repeats the cover letter, I'm going to be pretty annoyed. I'll probably read dozens (or more) of resumes for each job opening and if I have to read yours twice, once in the cover letter and once in the resume, chances are I'm not going to put your resume in the "A" stack.
Your cover letter should never be more than one page, and preferably no more than half to two-thirds of one page. It should have no more than about three or four paragraphs at the most. It should use short sentences, and each paragraph should highlight an important aspect of your experience, education, or skills that applies specifically to that job. It should be a "teaser" for your resume and make the hiring manager want to find out more.
The cover letter will usually be the first impression the hiring manager has of you, so make sure it's brief, articulate, and targeted towards that specific job. That's how you get your resume read more carefully, and that's how you increase your chance for an interview.