I use them for recurring tasks. My job entails overseeing a lot of details on an ongoing basis. (I manage a veterinary hospital.) I don't want to see the whole list all the time, but neither do I want to forget something, so I have entries such as "Do we need oil?" set to start again 2 weeks after I complete it, and "check the drive space on the radiology server" set for 3 months after and "Review open invoices and meds not picked up" set for a month after completion. None of these are time critical enough to warrant a due date, but there are literally dozens of this type of item and I dont want to have to scroll through them all every day.