When I write an app it's for that app to be used, to fulfil some purpose. I don't particularly care what the means to that end is, whether that's a Pi plus Raspbian or some other Linux, a Pi running Android Things, a Pi running Windows IoT, a Pi running Ultibo or Bare Metal, a Pi running RISC OS, or something else, an Android or IOS tablet or phone, even a PC running Windows, Linux or another OS.
What is ultimately chosen may steer how that app is, how it's implemented, but the goal is the app, not what it runs on or how it's achieved.
What's the difference between a Pi with a touch screen and a mobile phone or tablet with the same. Not much really. They are all surfaces for running apps on.