Half Bridge Low Side Mosfet vs. Flyback Diode
You typically choose a PWM frequency high enough to limit the ripple in the current through the motor to prevent torque oscillation and audible noise, and so the current during the off period won't decay to zero except at very low duty cycles.
   At 50% duty cycle, the motor will be running at about half full speed, and the current through the freewheeling diode will be equal to the motor current for half the time - so if you're passing 8A, you'd dissipate 3-4W with a 0.8 - 1.0V forward drop across the diode - enough to want to reduce the dissipation.

That 3-4W represents energy being dissipated from the field in the motor decaying, and if you can substantially reduce that, you reduce the rate of decay of current through the off period, so the on period can be shorter to regain the loss and maintain the same average speed - so the efficiency of the motor and drive improves. This improvement is most noticeable at low speeds (low PWM duty cycles) and reduces to zero at full speed. For battery powered equipment this gain in efficiency is worthwhile.