I have failure rule set up in some processes just in case the process has been cancelled by the user.
One example would be in the area of user permissions where you have a 'Enable Editing' yes/no attribute controlled by process.
If the user has the permission to enable editing, then the process would set the checkbox to Yes, EDIT the form, and when you save and exit, the process changes it back to No.
The failure rule comes in handy if the the user cancels the form without saving, leaving the checkbox value as Yes. This will leave the form open to editing to users without permission. In this case, failure rule can be set up to change the value back to No.