There is no real reason not to do online check in. If you register your credit card and give them your ETA and contact info, it may allow you to bypass the check in desk altogether. They text you your room number when the room is available. At the very least it will shorten the check-in process, since they will have all your paperwork ready for you when you show up. If you need to make a room request more specific than the options in the online check in boxes, you would have to phone the reservations number.
Your magic band will be attached to your
MDE account. You shouldn't have to do anything else to access anything. But since I don't trust them to be glitch-free (we've had problems in the past) I always ask the front desk to check that they should access my ticket, room door, charging privileges, etc before we leave the lobby.