The user has a task, that is due for a date. However, the user wants this task to appear in the 'Today' view earlier than the due date. Therefore, they set the scheduling date to the date they want to do it.
For example, I have a piece of work set. The teacher says it must be in on the 11/12/2016. However, I want to do it before that date, so I set the scheduling date for 02/12/2016, and it appears on that date whilst the due date is still left preserved.