I wouldn't write a cover letter unless the company explicitly asks me to, or unless I can add additional information that wouldn't be fit into the CV. For example, when I applied to Digital Ocean my cover letter was the correct place (in my opinion) to say that I've been a customer if theirs since 2014.

For the record, they were interested until I said I would have liked to work remotely (since they bragged extensively about remote work on their careers webpage).