Yes, it should clearly state why you should be hired and not the rest. It makes the impression showing the employer why you fit for the job, also acts as a searcher's opportunity in helping a potential employer to get to know your skills and experience and whether it fits what he wants. For more see http://jobsearch.about.com/b/2011/09/11/do-you-need-a-cover-letter.htm.
I personally don't think it's necessary. Years ago when you applied for a job you just filled out an application, and it went from there, either you got the job or you didn't. Today's way is a little complicated for some, and as far as I'm concerned, just a waste of paper. :)