Think of it as your intro post in a forum.
A cover letter should catch the attention of the reader (person screening the resumes) and give them a reason to review and take a closer look at your resume. It is much like the handshake when introduced to someone.
Redundant? Not at all. It is a critical piece of the application process.
How do you make one? You put your fingers on the keyboard and press the little letters to make words.
