Cover letter is useful when you are applying to a job that does not really fit you in terms of previous experience, e.g. career change.

You use the cover letter to quickly summarize how your present attributes can match with the requirements stated in the job ad.

If you are applying to the same job or job nature/title in different company, then yes, resume alone might be enough already, or sometimes just a recommendation can get you an interview - if the company is really desperate to recruit (not a good sign though!)