The pre-check in looks to be for the romance rewards so that they have the gift ready when you arrive, correct? Otherwise you don't have to do this, or did I miss something. I checked the FAQ and that is what I got from it.
It is part of the Romance Rewards Program where you tell Couples which arrival gifts you prefer. Theoretically, it is so Couples can have those gifts ready for you at check-in. If you don't do it, they will still get you your gifts, they might just have to be delivered to your room instead.