It doesn't make any difference either it's between the supply and the device or it's between the cable and the device it's still two devices.

By pushing the responsibility onto the cable it allows you to operate the cable directly from a USB port. So you can have things like electrical sockets with USB connections and you don't have to have chips in the sockets, because typically they're just dumb electrical interfaces. It also means that the device delivering the power doesn't have to be actually fully switched on, so you can recharge your phone from a USB port on your computer and you don't have to power the computer on. As long as there is an open electrical channel to the port the cable will deal with it all itself.

Also it's more efficient because you would have to have a control circuit in every single power delivery device, but this way you can have it in just the one cable, so now it is one chip for an unlimited number of power delivery devices.

The cable has to carry the negotiated power safely. It’s not unnecessary, it’s absolutely critical. I’ve personally seen and diagnosed the result of when this fails.

For your low power applications there is no need and the spec allows for that.