In cover later you concisely state why do you think your skills and experience are suitable for the position you are applying for. Also market your strenghts that is relevant to the Job.
I am not too sure about the "personal prefernce". I do not have one in my CV because I would like to keep my options open. I gues it could mean whether you like to travel, whether you prefer front end, back end development and/or integration work.
Also probably having a cover letter is a "personal preference". I do not have one but I make sure that the first page of my resume has a summary section highlighting my strengths and achivements. But it will be good to have one. [ November 06, 2007: Message edited by: arulk pillai ]