Basically, it allows you to preserve the relative position of Start and 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. 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 created with the following due dates:
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 remains the same, but the actual date itself changes dynamically based on the earliest Due Date.
You can also bring start dates along by checking this box when creating from a template:
Remapping Subtask Due Dates
If you're creating a template from a task that contains subtasks, you'll be given the option to remap the subtasks' due dates as well.
Check out this doc to learn more about remapping subtask due dates!