The roll back option allows you to undo changes to a page. You can make the page look like it did at any earlier point in time.
If you find that you made a mistake after publishing a page, you can remove the changes. You can easily go back to any previous version of the page - not just the most recent version. You never have to reconstruct a page to undo changes.
The Web Builder Express roll back feature is much more powerful than a simple undo feature you find in most services. With the undo feature of most services, you can only undo changes before you publish them, or undo the changes in your current working session. With the Web Builder Express roll back feature, you can restore your pages to the way they were at any point in history.