Cover letters are incredibly important. Typically sent as an email, you've got to pass this test just to get your CV or portfolio looked at. A good covering letter will tell someone who you are, why you contacted them and what you want. A bad cover letter, or no cover letter at all, and your application will most likely be ignored.
Make it personal. Nobody has time for a generic 'Dear Sir/Madam' because you didn't have time to find out who they were. Be quick to say this is who I am, this is why I am emailing you and this is what I want. Anything extra must be golden. Don't suck up but if there is something you truly admire about them, the company they work for or the brands they work with, let them know.
What makes a good cover letter?
Generic cover letters are common place but using a covering letter template is not the best way to go. Definitely take stylistic and structural cues from others and have your own template to personalise, but try to steer clear of replicating a covering letter from another person or using clichèd job words and phrases (such as "I'm passionate about media" or "I work well individually and as a team player").