I noticed that while a lot of companies do not require cover letters in addition to the résume to send, I still tend to find it in several cases.
In intern and trainee positions, it is much rarer, but in case of full- and even some part-time jobs, it's more prevalent.
What is the reason? What is so important in it, that a proper portfolio or reference can't prove? I mean, if I have either or both of these, then why do I need to attach an additional cover letter? Huge portfolio is a proof of dedication - or not?