let’s make up a situation and pretend that you’re writing a letter to a university you’ve applied to, enquiring about the possibility of deferring your course for a year.
Just as when you meet someone for the first time, you want to make a good first impression, so it is with a cover letter. A cover letter is your introduction to a prospective employer, a chance for you to sell yourself. A good cover letter will make a good impression on that employer...
Conventional greetings aren't always appropriate for the situation at hand; many times they may come off as too formal for the specific circumstances.