The importance of this can't be overstated. This is just my opinion based on years of interviewing and hiring devs, but...
The resume tells you the "what". What is the candidate's experience, what is their skillset, etc. Resumes get a once-over, mostly to see if the candidate is in the ballpark for the position.
The cover letter tells you the "why". Why should I hire this candidate over the others? Why does the candidate want to work here? etc.
The cover letter is the more important of the two. Assuming a candidate has the technical skills required, the things an interviewer really wants to know are softer: will they fit in with the team? Will they be happy in this role? And so forth. This is what the cover letter should be talking about.