When you're writing your cover letter, you might find yourself writing very vague sentences that sound a little something like, “I'm the best candidate because I am very dedicated.”
This might be true. But even Superman knows what he's good at and what he has yet to master. He can fly... but maybe he really stinks at baking muffins or directing traffic (because he's always trying to stop cars by stepping out into the middle of the street).
You have strengths, weaknesses and motivations too. And your cover letter is a place to start talking about them. When you're not specific about who you are, you miss a chance to deal with what makes you you—what sets you apart from a job candidate with the exact same qualifications as you.