A cover letter is an introduction to your résumé.
The purpose of a cover letter is to introduce yourself to the employer and persuade them to read your résumé. Its goal is to show them that you are the right person for their business or organisation.
Your cover letter should summarise your qualifications, experience, and positive work traits relevant to the position and explain how your skills, knowledge, and personal characteristics make you a good match for the job. A well-written cover letter also proves that you have strong written communication skills.
To convince the employer to read your résumé, tailor your cover letter to each employer. Sending out a generic letter when you are applying for a particular role can give the impression that you are not serious about the position.
Sending a cover letter, notably when it is not required, demonstrates to an employer that you are a motivated candidate.