A stylus and a touchscreen
One of Steve Jobs’s quotes goes:
If they need a stylus, they blew it
Does it mean that no device and no app should ever be used with a stylus? I do not think so and I suppose neither did Steve. The key word is need.
A stylus for a touchscreen is the same as a pencil for the “real life”. There is a place for it, but it is not required. Your fridge does not require a pencil to open it. Your bed does not require a pencil to take a nap on it. And so your smartphone does not require a stylus to make a call.
But when you draw something, you usually want a tool: a pencil or a brush — or a stylus, if you use an iPad. Styli are not included with the iPads, but make perfect sense as separate products.