Basically, it allows you to preserve the relative position of Due Dates on your tasks, without having to manually update them all.
For example, let's say we have this List of tasks:
Task 1: Due 10/1
Task 2: Due 10/10
Task 3: Due 10/25
I save this list as a template, then I want to use the template.
Let's say the new List I want to create is the same tasks and process, but for the next month (November).
remap due dates and for this list, I want the first task to be due on
I set the first date to 11/1 and then my tasks will be imported as:
Task 1: Due 11/1
Task 2: Due 11/10
Task 3: Due 11/25
See how that works? Each period between the Due Dates remain the same, but the actual date itself changes dynamically based on the earliest Due Date.