It is mandatory to have a cover letter with resume these days. You need to understand that when you are sending a resume to apply to any job a cover letter will be the first page that will reflect the resume.
You may find in the job advertisement that they are only asking for the resumes to submit but it is an unwritten rule to include a proper and very informative cover letter accompanying your job application. Whether it’s mention or not but the hiring department will expect that applicants should have a cover letter attached with them that will give a brief introduction of the candidates and their work experiences which will highlight the main resumes.
In fact it is the part of the job application that can convince H.R department to schedule a job interview for you. It is taken as a sign of laziness or ignorance, if you missed out the cover letter along with the resume. The main intention of attaching a cover letter will be to look for the summary of any communication that reaches him; the cover letter is precisely the portion of any job application that gives him a quick look at the person applying for the job.