I interpreted it as the resume shows your experience while the cover letter shows you know how to write coherently (plus gives you an opportunity to clarify anything on your resume)
The resume shows experience and the cover letter shows personality. If the job has any kind of soft skills a cover letter is a bonus, if the job is super technical it's probably not necessary. It also depends on the workplace.
If it is a job you actually want though I would recommend writing something. I'm on a smaller team and read all the resumes of applicants. I actually read them because I'm going to be the one contacting, interviewing, and working with them. I absolutely read the cover letters and give a small bonus to people who include them.