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.