My sense is that "devices" you hook up to a computer are themselves entire computers with fancy high-level protocols to control them. Sure the Pi, unlike a norm

My sense is that "devices" you hook up to a computer are themselves entire computers with fancy high-level protocols to control them. Sure the Pi, unlike a normal computer, has its GPIO ports. You can use those to directly control hardware that is not smart (or is only a tiny bit smart). But still you run out of ports really quickly, managing those ports isn't easy, and if you are controlling things that need more power than the GPIO ports can provide then you'll need additional circuitry. You usually need additional circuitry. The obvious solution is to create higher-level hardware that the Pi can control without, say, pulsing on a regular interval, or without having to provide the full power requirements via GPIO. An Arduino board is a modular way to do exactly that. The Arduino then becomes the "device".