I always thought of a cover letter for clarifying something on your resume. Ex: you’re changing careers or industries and out want to clarify why your experience is relevant. So, I don’t do them for every application but in certain situations.

When I get them from new grads I delete them. Experienced people or weird resumes I might read if borderline.

Same. They already have my resume and application for the job, I’m not writing a whole page groveling and begging them to hire me.

It’s worth writing a generic one.