The non- app is used on Aurora, but not needed for booking mdr restaurants and entertainment.
You can use it for booking speciality restaurants (I think, never use them), booking excursions, accessing you on board account, checking your calendar which shows the itinerary and any bookings you have made. It also has The Horizon programme and that days edition of Britain Yesterday (Today) which has more pages than the printed one.
I find very useful and better than carrying multiple bits of paper around and having to go to reception (and more paper) or cabin TV (not on Aurora) to check your account.