Cover letters are simply a letter of introduction.
In these letters you are explaining who you are and why you are writing to the resume reader. It is a matter of courtesy that you introduce yourself when you are requesting something of them, in this case an interview. My rule is - if you are not shaking hands with the hiring decision maker and introducing yourself, then you need a cover letter to introduce you. If you are shaking hands with the hiring manager, you are introducing yourself verbally and requesting an interview. In this case, hand them your resume without a cover letter.