This varies by country, but for the United States, the law requires you to honor opt out requests. A link is one mechanism to honor such a request. Another way

This varies by country, but for the United States, the law requires you to honor opt out requests. A link is one mechanism to honor such a request. Another way is to remove a recipient from a list after they reply stating their wish to be unsubscribed. From a practical level, it is worth including an unsubscribe link just to avoid having someone mark your email as spam after not being able to find the unsubscribe link.