I have no clue in general. When I go through applications, I go through all the material and anything else I can find about the candidate, and cover letters are sometimes helpful. Put yourself in my shoes:
- If the cover letter is a rehash of your resume, it's a waste of your time and mine. It certainly isn't helpful to your application, and if I have too many well-qualified applicants then it might be harmful.
- Poorly written cover letters suggest that the applicant doesn't care much about this specific opportunity, it's some sort of AI/oversees/... scam, or the applicant can't write well. They're very helpful for me when there are already other data points suggesting identity theft or similar automation (nail-in-the-coffin material). Otherwise, they're not necessarily a negative, but it's rarely advantageous to advertise a lack of some skill, and it does disqualify applicants from some roles.
- Some cover letters are especially compelling. Suppose your resume just has you as a pizza delivery driver, but your cover letter goes over the app you wrote and the data science behind it to optimize your hourly earnings. Suppose your work history is in web tech, but you're actually better at low-level optimizations and are applying here because you think that skill set is a good fit. I prefer varied backgrounds anyway, so you'll probably get some form of screening interview unless there's enough other evidence that you aren't qualified (e.g., junior experience for a senior role) -- I try to bias toward giving everyone a chance while not wasting too much of anyone's time, and I'm fine having a busier calendar to make that happen.
None of that helps you get the job though; it helps you get an interview. If your experience is that you're well qualified and usually land the job once you get into an interview, and if your cover letter has some information your resume lacks, you should definitely add one. If you normally struggle through the interview process, it'd be surprising for you to have an honest cover letter which would help you land the job in the first place, so I probably wouldn't bother.
- If the cover letter is a rehash of your resume, it's a waste of your time and mine. It certainly isn't helpful to your application, and if I have too many well-qualified applicants then it might be harmful.
- Poorly written cover letters suggest that the applicant doesn't care much about this specific opportunity, it's some sort of AI/oversees/... scam, or the applicant can't write well. They're very helpful for me when there are already other data points suggesting identity theft or similar automation (nail-in-the-coffin material). Otherwise, they're not necessarily a negative, but it's rarely advantageous to advertise a lack of some skill, and it does disqualify applicants from some roles.
- Some cover letters are especially compelling. Suppose your resume just has you as a pizza delivery driver, but your cover letter goes over the app you wrote and the data science behind it to optimize your hourly earnings. Suppose your work history is in web tech, but you're actually better at low-level optimizations and are applying here because you think that skill set is a good fit. I prefer varied backgrounds anyway, so you'll probably get some form of screening interview unless there's enough other evidence that you aren't qualified (e.g., junior experience for a senior role) -- I try to bias toward giving everyone a chance while not wasting too much of anyone's time, and I'm fine having a busier calendar to make that happen.
None of that helps you get the job though; it helps you get an interview. If your experience is that you're well qualified and usually land the job once you get into an interview, and if your cover letter has some information your resume lacks, you should definitely add one. If you normally struggle through the interview process, it'd be surprising for you to have an honest cover letter which would help you land the job in the first place, so I probably wouldn't bother.