Most ATS platforms don't score cover letters the way they score resumes — the parsing and ranking logic is almost always built around the resume document specifically.
So why write one at all?
Because the human reader still might. A cover letter is your chance to explain context a resume can't — a career change, enthusiasm for a specific company, or a gap — without cluttering the resume itself.
When it's worth skipping
If the application explicitly marks it optional and the role is high-volume (large retail, hourly positions), a generic cover letter rarely adds value. Spend that time tailoring the resume instead.
When it's worth writing
Career changes, referrals, or smaller companies where a human is very likely to read every application closely.