I just checked in with a recent client, Jane, who – at the start of our project – was as anti-cover-letter as anti-cover-letter gets. She was one of the many people out there who believe that a cover letter is a "throw away" item – something that needs to be done but not very mindfully. Early on, I explained to her (as I often do when clients ask) the importance of a cover letter.
First off, always submit a cover letter with your resume unless a company/organization specifically asks for only a resume. An ideal cover letter can take many forms to be sure, but it's main goal should usually be to motivate the reader to take a close look at your resume. Moreover, I typically like to see a cover letter that gives the reader a bit more information – some context that will help them to make further sense of the resume. Oftentimes, a cover letter becomes more important once the list of job candidates has been whittled down. That's when the "extras" that a cover letter provides can make all of the difference.
Jane let me know that she believes it was ultimately her cover letter that got her a job. Her new employer told her that it expressed a level of interest and sincerity that set her apart from other applicants. Cover letters are still a big part of the job-search routine. Use that fact to your advantage.