A cover letter can provide a potential employer with information that cannot easily fit on a resume. For example:
A cover letter also gives you the opportunity to demonstrate your ability to write clearly. Your correct spelling, proper grammar and complete and informative sentences, can be an additional motivator for an employer to ask you for an interview.
You should always include a cover letter with your resume. Revise it each time, to be relevant to the position you are seeking.
You should always address the letter to a person (not Sir/Madam, To Whom It May Concern). If you don’t know the name of the hiring manager, you can use the Human Resources Manager’s name. It’s easy to find this on the company’s website.