A to do list is a really clever way to look into progress you have made in a day so, it is not just about completing the task it is about looking into the progress you have made by glancing at completed tasks. You don't get default features to see the competed tasks to enable that you have to go into settings and enable that and if you are not interested in compiled tasks anymore you can't just delete those tasks for that you have to select each task individually and then delete it.