Cover letters are ineffective and irrelevant in today's job market. In most cases, the hiring manager will not even see it. It will be screened out by recruiters and HR departments. I haven't used a cover letter when applying for a job in over 20 years.
However, there could be some exceptions. For example, a smaller company that isn't using a recruiter and doesn't have a full time HR department may take a look at a cover letter. Another might be if you have met someone at a conference or the like and want to remind them of this encounter.
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