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If you start doing something you expect to accomplish in one sitting, but are interrupted, you can add a nested task to it to show you where to resume. An advantage of doing this over simply modifying the original task is that you've still got the original task there when you look back at the project later.

Another common way the nesting of tasks is useful is preparing for some activity. A parallel group of tasks up front to collect signatures or feedback or parts or tools, followed by a sequential block of tasks to do the work. The parallel group allows all those tasks to be active when you encounter the colleagues or merchants, whereas doing it with a completely sequential project means you're likely to do it inefficiently if you didn't guess the optimal order.