Updating a data extract sometimes goes wrong, causing workbooks to malfunction. However you can't go back to the previous data extract (when everything was still working OK in most cases).
Tableau Online only stores the revision history of the XML. So, basically everything outside the actual Data, because otherwise they would need to store a lot of data. However this means that if a mistake occurred in the actual data, you may need to rework and republish workbooks to fix it (been there, and this can be lots of work).
Tableau stores the data extracts (so the data itself) of two versions, the current one and the previous extract. This means that you can restore the actual data, not just the XML. This will mean more data will be stored, but Tableau becomes much more robust for data extract errors that may occur. It prevents frustrated customers and saves time of customers and helpdesk staff.
How does it work?
1) You choose the previous Revision of a Data Source
2) You select "Restore"
3) You get a short explanation what will happen and you need to confirm
4) The previous revision becomes the current Revision with a new number