Not long ago I polled a healthy collection of recruiter buddies. Do they read cover letters? The overwhelming response was "No!"
OK, then............should you bother with one?
If you're submitting a resume that no matter how hard you try just doesn't make it intuitively obvious why you're a fit for a position, perhaps you need a cover letter. Maybe you're making a career change. Maybe you're interested in taking a step back on the proverbial corporate ladder. Maybe you've always worked full-time, but are very interested in an employer's part-time position. Maybe you'd like to be considered, but can't relocate just yet and would like to have an exploratory conversation now if possible. Maybe the company has stated they only want local candidates and you're not local, but are relocating to the area within the month. Write a cover letter to explain yourself. Do so succinctly and matter-of-factly. Make a rational, plausible case for why you should be considered for the job.
One thing a cover letter should never do is rehash everything that's already on your resume. If Sally recruiter is scrambling to keep up with all the resumes in her stack, the last thing she needs to do is re-read your resume in cover letter format.
Make sure any cover letter you write is succinct, gets right to the point, and is grammatically correct with absolutely no spelling errors or type-o's. A cover letter should probably never go over 1 page. And, it should be written in a respectful and considerate tone without being pleading. Think of this as simple business-to-business communications, with you being one of the businesses.
And, if you do write a cover letter, do not send the same boiler-plate cover letter to all employers. Believe me, recruiters can tell. Talk specifically about the job you're trying to get.
If you submit your resume via e-mail, your e-mail submission is another form of cover letter. Abide by all the same "rules." But, if you want your cover letter to become part of your Applicant Tracking System record, use a Word attachment instead, naming it "Cover Letter."
If you have a reason you're going to need a cover letter, write some practice drafts. Show them to friends and get feedback. See what kind of reaction you get. Hang on to those for future reference that seemed to appeal to the most people.