Cover letters on a resume can have a powerful impact if written correctly. In writing a cover letter you are telling the potential employer about your experience, who you are, and what you expect to achieve with their company. You can write a cover letter that is too long and that covers too much information to be useful to you. For example although you may have alot of experience it is not always a good idea to put this all in your resume. It may come off as something else in context to the employer like working only a small amount of time at each job. You should write several sample cover letters before you submit your final.