I don't know how it is in the US, but in Europe, a newsletter by law needs to have a clear way of unsubscribing by providing a link somewhere.

Why unsubscribe?
1. For the environment. It is dirt cheap to spam e-mails around. But the amount adds up in electricity and traffic.
2. For your own privacy.
2a. If you use a commercial mail provider that scans your mails, then each time you get one of those, it is scanned and the profile on you grows.
2b. In case the sender of the newsletter is ever hacked (chances are it will at a certain point), your address will leak to the crackers.
2c. The sender has your address and can (and probably will) trade it for money. In an ideal world (we all like to think that hope dies last) they delete your address when you unsubscribe. If not, it's probably a violation of GDPR.