Since the checked box is immediately usable (unlike Delve Too Deep which let you shop for more only after clearing the scenario) you can :
- Make any Customizable with single box choice suddenly usable while playing the 1st scenario where you are not supposed to have any XP yet.
- When you are missing only 1 box to get a key upgrade from all the XP you got in your previous scenario, or you may intentionally check boxes such that it is missing just 1 and use remaining XP for regular card purchases instead. Carrying Refine into the next scenario and bet for it to check the final box can stretch the limit of how fast you can get good in first half of the campaign.
- Maybe able to stretch this more with In the Thick of It, for example checking 3/4 box of Pocket Multi Tool Spring-Loaded then bet on Refine to check the final box. Or starting Runic Axe with Saga and then use Refine to add one more inscription inside the scenario to go with it.
Also, if you are planning to add in some Customizable cards into the deck later together with boxes checked (e.g. would rather use deck space for something else than 2x Summoned Servitor in the 1st scenario that cannot do anything), then you do not "own" the card just yet, and cannot use Refine to check the box. (However 1x Summoned Servitor is enough to receive Refine.)
(Ownership and Control : A card's owner is the player whose deck (or game area) held the card at the start of the game.)