It is very common to use graphical workflow to help facilitate some change management process within your organization. One common requirement in change management is to be able to cancel or close the change request at any time during the process. "Simple", you say. "Just allow the user to change the value of the 'State' field to 'Closed'." You would not be incorrect in saying something like that, but you would be forgetting about part of the problem with closing or canceling a change request or other task ticket.
When implementing the Change management process in Service-now you'll probably encounter a scenario where your entire change workflow (including all tasks and approvals) needs to be reset. The first option to consider (assuming you're using the graphical workflow engine to manage the tasks and approvals) is the Rollback workflow activity. The rollback activity works great for a lot of scenarios, but what if you don't have a defined point in the workflow where everything should be rolled back? What if the rollback (or reset) can happen at any point in time? There's not really an easy way to handle this within the workflow itself since you would need to check for a rollback condition at an infinite number of places.