Apps would not be useful if there was no way for either the user or the system to input data. The very definition of an app is to fulfil a purpose for the user, and the only way to let the app know what to do is to provide it with input. Almost all apps require some input, ranging from word processors with hundreds of key strokes and mouse clicks to screensavers, which close as soon as the mouse moves. All apps take input, process it, and output the result in some form.
For modern mobile devices, the primary form of input is touch, and on some devices, there is only one hardware button, the power button. Initially, Android devices were built with keypads, but now, almost all devices are built with a large touchscreens.
A touchscreen is actually very limited, because they can only respond to input when a user actually touches the screen. This makes processing the touch events the real source of input. Depending on the pressure the user applies, how many fingers the user uses, and...