The Apple II had slots and was thus highly hardware-customizable. The ability to add specialized hardware to a general-purpose machine gives it that much more flexibility, whether it's additional storage, data acquisition, additional network interfaces, machine controls, etc.
Sure we have things like Thunderbolt today, but there's a lot to be said for putting the specialized stuff inside the case where it isn't cluttering up the desk and nobody can just unplug it and pocket it.
It was extendable, but not modular. You couldn't just buy different parts and put them together into an Apple II. It was sold as a whole, that you could add a few bits to.
Sure we have things like Thunderbolt today, but there's a lot to be said for putting the specialized stuff inside the case where it isn't cluttering up the desk and nobody can just unplug it and pocket it.