Haven't these things been out for years?
You have no idea how many application on iPad can not run via cable. I work with Midi and I have downloaded many instruments for my iPad and any of them can transmit midi (and forget about audio) via USB connected to your iMac. iPads are pretty much back in the 70's regarding midi desimplementation when people used to use control voltage to make electronic instruments talk to each other.
Apple just do not care about opening the channels for the cables. You have to do everything via Wi-Fi.