The electric motor is biased "ON" just a bit to provide the "NORMAL" tendency found in traditional cars with autopmatic transmissions to CREEP FORWARD just a bit.
If you stop your Prius o a bit of an upward incline it is likely that it will not roll backward. This is like the "HILL HOLD" feature found on some other cars. The Prius doesn't roll backward because the electric motor is holding the car and expends battery power to do so. Under these conditions note the energy display and you'll see that the electric motor is powering the wheels. If you put your foot on the brake the motor shuts off.
Not a giant savings but if you spend much time stopped on hills it could save some battery power to use your brakes. Some drivers with automatics use their accelerator pedal to hold their car on a hill by engine power. In a Prius this would waste electrical energy.
Pat 
If you stop your Prius o a bit of an upward incline it is likely that it will not roll backward. This is like the "HILL HOLD" feature found on some other cars. The Prius doesn't roll backward because the electric motor is holding the car and expends battery power to do so. Under these conditions note the energy display and you'll see that the electric motor is powering the wheels. If you put your foot on the brake the motor shuts off.
Not a giant savings but if you spend much time stopped on hills it could save some battery power to use your brakes. Some drivers with automatics use their accelerator pedal to hold their car on a hill by engine power. In a Prius this would waste electrical energy.