Allow you to express your individuality, abilities and compatibility to potential employers. When you submit your résumé, a cover letter should accompany it.
Employers appreciate receiving thank-you letters and in some cases where the competition is especially keen, a thank-you letter may give you an edge over candidates who fail to send one.
The correspondence done in a job search involves more than just the cover and thank you letters. Sample PDFs included.
Do you need stronger descriptions for your job duties? Have you used "Responsible for. . . " too often? Try this list of action verbs to help you along.