I've been a hiring manager, I simply don't see the value in them for most job postings. Maybe if you're trying to get your first job out of school, or you're trying to pivot into a new industry, sure. If you're applying for, say, an accountant role, you've been in the industry for 20 years and the position you're applying for is largely in line with your experience, just a step up the career track or a change in volume, I don't see the point.

Heck, I see retail jobs asking for a cover letter sometimes to be a cashier and stock merchandise. Waste of time for everyone involved, and only serves as a filter for the employer to see who's desperate enough for a job, or passionate enough about whatever is being sold, that they'll put up with whatever.