Hmmm, a really good question! I would say it depends on the job vs position (career). If a simple task job like working at McDonalds, then, no, a cover letter would not be necessary. A basic resume is all that is needed with just the facts and dates pertaining to potential employment there.
If going after a position/career then the potential employer would want to know if you are capable of the communication skills that would be of benefit to the company. The potential employee-through the cover letter- lets Human Resources know they are able to represent the company in person (actual interview) and on paper. This is a win/win situation.