Many job seekers treat cover letters as a necessary evil. They put some effort in, but mostly use a template that gets tweaked for specific applications.
It's easy to see why people would do that. Your resume details your experiences and why you're qualified, so the cover letter seems more like a formality before the hiring manager gets to the main course.
Thinking of it that way can hurt your chances of getting hired. A good cover letter can help you stand out, and can also get the person making the hiring decision decide to give you an interview even when your resume might not have.
What makes a good cover letter?
At the bare minimum, a good cover letter feels like it was written in response to the specific ad being answered. It should address any questions from the job listing that aren't clear from your resume.
That should be accomplished through specific examples. For example, if the ad calls for "strong sales management skills," don't respond by saying that you are "an experienced sales manager." That should be readily clear from your resume. Instead, offer details of how sales improved under your leadership at your current and previous positions.
Your cover letter is also a chance to explain experience that may not be obvious from your resume. Perhaps you have managed a multi-lingual workforce, or maybe you have coding skills that don't relate to a previous job. Find a way to work it in that ties into what the employer has asked for in the job listing.
What can a cover letter do?
A cover letter can be used to convince the person doing the hiring that you should get an interview for a job you're not qualified for on paper. To make that happen you have to show how your experience, while not what they are asking for, makes you a good fit.
For example, for years when I first interviewed for business writing jobs, I cited my experience actually running businesses. In those cases, my writing credentials were not in question, but I lacked specific business writing and editing experience.
By noting that I had run a retail store and a factory, I made the argument that I brought a perspective people who had followed a traditional path lacked. That got me interviews at places that may have otherwise overlooked my candidacy. Ultimately, using my cover letter to crack open the door led to the jobs which helped me get to my current career.
It's an opportunity
Think of your cover letter as an opportunity to talk the hiring manager into giving you an interview. Address their needs and make a case for yourself. Don't be silly or overly personal, but do allow some personality to shine through.
Resumes are cold -- they list your accomplishments and work history, but they don't show who you are. Your cover letter can do that, and that could be the difference between being passed over and getting an interview.