Though there are opposing camps on whether cover letters are required, it's best to read the job ad carefully and send one if it's mentioned.
Without a cover letter, the candidate is counting on me to infer - in just a few seconds - how the experience on the resume is related to the company's needs. The cover letter is invaluable in helping me understand how the candidate's experience is a match.
Our impression from hiring managers is that they are not reading cover letters. We are more in favor of short email introductions that succinctly highlight a candidate's strengths and weaknesses in relation to the job opportunity. We also strongly advise candidates to add a detailed technical skills section to the top of their resumes.
Either way, you have to write something and be specific about the position you're seeking and why you'd be a good match. A post at 6FigureJobs.com points out how a cover letter by a 6-year-old hits all the right notes. It published the handwritten letter by young Sam Pointon, who wanted to become the new director of the National Railway Museum in the UK. His letter:
Dear Mr. Tucker,
Application for director
I am writing to apply to be the new director of the National Railway Museum. I am only 6, but I think I can do this job. I have an electric train track. I am good on my train track. I can control 2 trains at once.
Advice from the article: