ME "checks you in" when you check in at the airport. Therefore, the resorts get a little "heads up" when you are coming. If a room is either open early in the morning, or gets cleaned between the time you check-in at the airport....you are listed as "already waiting" for a room. It's a little "perk" that WDW is giving to those who use ME.
Many people have suggested listing yourself as using ME, and checking in, even if you then pick up a rental car.
Beca