Writing Letters for Cold Canvassing (IT) Jobs?

jtan163 asks: "I'm trying to help a friend write job applications for cold canvassing jobs in the IT industry but we're quite stuck. His CV/Resume shows his skills and employment history and even to some extent, what he is looking for. So, what do you put in the letter for cold canvassing IT (and probably any other field) job applications? With cold canvassing, is the letter really important or is it just 'fluff' so the CV/Resume doesn't get lonely in the envelope (electronic or otherwise)? We'd love to hear about what has worked or not for you. Or, if you happen to be a job application consumer, what you look for in, or at least what would make you consider (or at least not throw out), a cold canvassed application?"