A cover letter, as used in the job_hunting context is a document that is supposed to provide an introduction to the resume or curriculum vitae, which are the basic document employed when marketing one's own services to a potential employer. In the traditional way of doing things (when job hunters used 'hard copy' applications), the 'cover letters' was typically attached on top of the curriculum vitae or resume, thereby typically serving a cover in the literal sense of the word, to the Curriculum vitae or resume; and it is probably from this fact that it probably got its name 'cover letter'.
We will all go through writing a business letter at least once in our lifetime. It does not matter if you are an employee, an owner, a stay_at_home mom (or dad) that has her own business at home; we all have to go through writing it. It cannot be avoided!