Probably nothing (it's a running joke around here). But as I said on the June Roll Call thread, we all do it anyway, just in case it might get us to the Sexy Pool 5 minutes sooner. Edit: Just read the express check-in confirmation email, which indicates we are checking in today and checking out tomorrow, which is definitely not what I inputted. Perhaps a case in point about how useful the express check-in feature is!
For us, besides possibly getting thru a little quicker, it’s the security that our reservation is in their system... we’ve heard random stories about people checking in and resort not having a room and trying to divert to a different resort.
As others have said, probably nothing, but we have requested room locations with it and always get the location we request. And it gives you a warm feeling inside that you're within range and who doesn't like a warm feeling inside, other than prison inmates?