To use a 3.5mm jack headset you need to add power amplifiers inside the phone, do proper impedance matching and hope that noise in the path does not corrupt the data. But sending power and digital data solves all these problems, the wires don't need to be as well made either!
But the noise susceptible analog signal does not have to travel a long path, digital signal does. As such the power amplifiers do not have to be as power consuming/over designed. Moreover, you can do other stuff with the headset like taking multiple button inputs without additional wires.
