Excellent when you are waiting to hear back from someone or you're told to follow-up in X days for documentation before you can complete the work for the task/sub-task and move on to the next. This isn't always the due date. For instance, I wait on paperwork from the state. I have a range of 7-14 days. My due date is in 14 days but I want to start following up on the 7th day. Or I am waiting for information from someone so i can submit the paperwork to the state. I want to add a follow-up date to remind me to check with them prior to submitting the paperwork but I still have a due date to submit said paperwork.