A well-written cover letter gives you a chance to sell yourself to the employer in a narrative format, and explain why you are an ideal candidate.
That's why you definitely should write one.
You will make it clear that you are highly interested in the job, and that you want the job enough to take the time to write a cover letter.
Long letters carry a huge risk of getting just a glance, but no hiring manager minds reading a brief half-page story. Especially if the targeted resume of the same candidate is put together perfectly.
A cover letter is your opportunity to explain how you solve problems. It helps to show your personality and reinforce your professional image. Finally, a cover letter advertises your strengths and convinces an employer that you can benefit the company.