Depends on the company. Most of the big drilling companies (and probably most places that aren’t backwater Mom and Pop companies) these days use HR software looking for key words on your resume and won’t even look at the rest of your submission unless it passes that test first. I do a cover letter regardless but I’m a bit old school on that maybe.
I always try to walk in and talk to someone. The cover letter is only for use in jogging someone’s memory of me or if I fail to get past the receptionist. Getting past the receptionist is key. Its also sort of a sport: I have been defeated by some social kung fu masters once or twice. Respect the receptionist.
I think a cover letter is important. The resume tells who you are, your experence and education. I used the cover letter to say how I intended to apply thoses skills in the new position.
Only used a cover letter one time, in the union it’s possible to move to C/M without much scrutiny from the office but moving to master means working for the office instead of the working for the captain. The office drones needed a good justification to show the higher-ups.