Scammers use fake ‘unsubscribe’ spam emails to confirm valid email accounts to be used in future phishing and spam campaigns. These emails simply ask if you wish to unsubscribing or subscribe. The emails do not explain what you are doing and are being used by spammers to verify if the recipient’s email is valid and susceptible to phishing scams and other malicious activity. If you receive an email that just simply asks you to subscribe or unsubscribe, mark it as spam.
Source: https://www.bleepingcomputer.com/news/security/watch-out-these-unsubscribe-emails-only-lead-to-further-spam/