The text backup is used by email publishers alongside the HTML creative. The text backup is the plain ASCII type version of the HTML creative and will be displayed instead of the HTML creative for users that do not have HTML enabled email.
Text Backups are another way to optimize your campaign and ensure greater visibility of your campaign to more viewers. Text backups help improve emailer deliverability.
An unsub link and a physical address are required by CAN-SPAM for all email creatives.