We use a fixed gear on our studio bikes because it allows the rider to pedal more smoothly in all situations. The fixed gear helps to improve the overall user experience in several ways:
When riding while standing, momentum from the fixed gear helps the rider at the very top and bottom of the pedal stroke. On a bike with a freewheel, this can create points of disengagement, which can result in an uneven feel.
When riding at high RPM, the fixed gear keeps the pedals in time with the flywheel and allows the user to pedal continuously without losing rhythm. With a freewheel, the pedals can easily fall behind the flywheel when it is spinning at a high speed. When this happens, pedaling becomes uneven and awkward until the flywheel slows down.
In times of high resistance and lower RPM, the momentum generated by the flywheel helps to pull the cranks through the pedal stroke (again, primarily at the top and bottom) where positive torque is not being applied by the rider.