The checkout header fields can be added to the checkout, but sometimes these fields affect the basket calculation and a customer should enter the value earlier in the ordering process, to get the proper details of the order.
Think about for instance a partial delivery checkbox or the requested delivery date that may alter the prices or availability of a product. Making it configurable where you want to show the checkout header field (basket or checkout) any field that you can currently add to the checkout will be possible to add to the basket instead.