Every time you apply for a position, you need to write a cover letter to accompany your résumé. A cover letter allows you to formally introduce yourself and explain why you are the best candidate for the position. In it, you will highlight your experience and skills as related to those listed in the job description. Most cover letters follow the same general format but should be specific to each individual job for which you apply. If the job description changes, reasons why you are the best candidate change.
Keep in mind that your cover letter should reflect your experiences and abilities that make you uniquely qualified for the job!