If you can save someone a significant amount of time or hassle your app will work. A great example of this is the Sam's Club app: as you shop you can scan the barcodes of the products you're buying and then make the purchase right on your phone which presents a QR code for the cart-checker at the door to scan to verify you're not stealing. This app allows the shopper to skip the check-out line, which saves time. Any app which speeds up a slow process you have to do repeatedly is likely to be a winner.