Still don't get it to be honest.
This is the text from the popup:
"Automatically replaces expired or self-signed SSL/TLS certificates with free valid certificates from Let's Encrypt. Covers each domain, subdomain, domain alias, and webmail belonging to the subscription."
So, if the domain, webmail and mail is secured with a Let's Encrypt certificate, this option isn't needed? Let's Encrypt certificates are renewed automatically after 3 months, right?