I am using Sendwithus email templates, I see <unsubscribe> tags on plain-jane template I want to know how tags work to unsubscribe the list and what is the advantage of its?
If you're using sendwithus, you'll rely on your ESP (email service provider) for unsubscribe functionality. An unsubscribe tag is a special bit of text that your ESP recognizes, and will transform into an unsubscribe link before actually delivering the email.
You'll need to check the documentation for your ESP to determine how to format the tag. I recommend setting up a snippet in sendwithus so you can easily include the tag in templates via {% snippet 'unsubscribe' %}.
As an example, if you're using SendGrid as your ESP, you can create a tag as
<div>
<a href="sendgrid-unsubcribe">Unsubscribe from our emails</a>
</div>
See https://support.sendwithus.com/delivery/unsubscribes/ for another reference.