In the history, you can see who completed the task on Cassie’s behalf.
Remember the Workflow Status page? It’s also important to know if (and when) an outside participant has been included in your workflow. If later on, for example, there is a concern about whether the file was approved by Cassie or not, people can refer to this page and get their answer. They'll see in the history section that a task was created for Ellen on behalf of Cassie. They can follow up with Ellen at that point if they need to.