The Planned date is when you plan to start working on a task. By default it is considered as already workable from now on.
The Due date is optional. This is the day the task must be completed. This information is not always mandatory, or even precisely defined. For many everyday actions that we finish today or tomorrow does not change anything, so it's useless to invent deadlines when they do not exist because it will require you manage them :)
If work does not start right away, you can schedule it or snooze it to a desired date:
The defined Planned date is then displayed:
When there is a due date, you can specify it (and attach a reminder to the task's assignee)