Some cars are going with entirely electrically actuated brakes, either inboard on on-hub, compared to the E-Tron which uses traditional hydraulically actuated brakes. One uses an electric motor to wind something to tighten the spring clip by pulling it that then pushes the pads to the rotor and the other uses pressure to overcome the spring by pushing the spring to compress it and push the pads to the rotor. I'm guessing Audi didn't go with entirely electric brakes because they have a reputation for being harsh and difficult to modulate with the pedal, and Audi is supposed to be both a luxury and sport brand where pedal feel is important.