Sure they want user data to observe people's purchasing habits. But they already have that if you always use the same debit or credit cards like most people do.

But the more people use the app, the less cashiers they need and the less ordering kiosks they have to install. Plus customer satisfaction goes up because you can order ahead and your food is ready when you arrive. And getting used to the discounts means you probably won't switch to Burger King or Wendy's.

I think additional user data is a relatively minor part of it.