Cover letters don't get enough respect in the world of job searching. Many people do not want to include a cover letter, but most employers look to the cover letter as a sign of respect and formality that does the work of piquing the interest of the employer to consider the resume. Does the job searcher understand the demands of the job? Have they conveyed their skills in a way that summarizes how and why they want to work in this particular job?
One way to rapidly scan through numerous applications is to exclude anyone who has not bothered to prepare a cover letter. Secondly, if the cover letter is a standard letter that doesn’t address the job in particular, an employer can screen out people who are not willing to go the extra steps to get hired. Employers see this as an expression of your approach to the world and to your future work.