Imagine you want to model an escalation of a user activity. After a specific timeout a transition should be taken to deal with the escalation. The transition might lead to a user activity assigned to someobody else or to a different role or more important to the next steps performed automatically without a reaction of the user.
When the timeout event fires the transition is taken and the execution moves on to the next activity. However the task remains in the user's task list who had the task assigned. The task was not completed. Deleting the task will not succeed since you cannot delete a task which belongs to an execution. Completing the task will also fail since the associated execution does not exist anymore but was already removed from the database.