Karen Kelsky explains at The Chronicle of Higher Education:
Question: I’m preparing my job documents for the fall and looking for ways to economize. Can I just write a really short cover letter since all the information I would put in a letter is already on my CV? The cover letter feels redundant.
And the reason for that is — they are two different documents. They have different functions and are designed to help the search committee ascertain distinctly different things. Summer is a good time to go over the basics of both documents as candidates prepare for a new academic hiring season.
Read the rest here.