Not sure why this isn't a standard thing but when creating a config product with different colours and sizes often the price is the same for all. With this checkbox, variations will have the price greyed out and copied/updated from the main configurable product.
Without it now, you have to enter each variation price manually at the beginning and remember to change each one manually, so much more work than necessary and it leads to errors.
(We have had to open a ticked for a missing variation, which was not showing in the admin even as it had no price?)
Thanks - Adam.