Only send a cover letter if they ask for one. When they ask for a cover letter, it’s usually because your resume is going to be screened by software or a low level HR clerk, neither of which is equipped to interpret your resume to determine if it matches the job description. Instead, they want a shortcut.
The purpose of your cover letter is to give them that shortcut. Therefore, a cover letter should be a clear, concise and coherent summary of a match between you and the key/essential requirements of the position.
Think of it as a fact-matching exercise
List their key requirements in one column then correlate your experience and skills in metric terms with those requirements. For example, if they require a degree in a field, name your degree and year of completion’ if they ask for X years of experience with a certain software, provide them with your number of years; if they want to know what kinds of projects you’ve worked on, be specific (name of project, duration, objective, your role, budget, results); and so on. Once you’ve done the two-column exercise, you can formulate a one-page cover letter.
You then refer them to your resume for details. The cover letter is not for you to emote about how much you admire the company and what a great candidate you are. The purpose of the cover letter is to give them a good reason to read your resume where you demonstrate your “job specific knowledge” and provide evidence of why you can perform the required “duties and responsibilities”.
Cover letters and resumes need to be customized to a job posting. They are marketing documents that help differentiate you from other competitors. In order to optimize your chances of breaking through the glut of applications and rise above the noise, get professional help!