The Fandango app for Android isn't very good. It's just a web wrapper for the most part, and half the time when I try to buy tickets, the purchase times out and I have to try again. But now, there's one major reason to use it over the mobile site - Android Pay.
Now when you buy tickets through the Android app, Android Pay shows up as a payment option. I just tried it out by buying two tickets to Spider-Man: Homecoming and it worked great. Much faster than PayPal - it's just like using Android Pay in a physical store.
As the late Billy Mays would say, "But wait, there's more!" If you buy tickets with Android Pay and use promo code AndroidPay1 at checkout, you get $3 off your purchase. Google says there will be a new promo code every week until July 5, so just check the Android Pay promotions page for the most recent code.