The CV is a list of your achievements
The cover letter/email is a story of how you reached those achievements specific to the job that you're applying to.

At early stages of your career maybe it won't make much sense but at the middle/later stages it paints you as a human

The world is full of AI though, so sad to say I don't know how a cover can make a difference unless it goes to a human. If you read the viral posts on LinkedIn, most will say that the cover email is only relevant when you reach the hiring manager directly