Originally Posted by supcom

Unless you have poor brakes, the front brake is all you need for the shortest stopping distance. As you apply the front brake, your weight distribution moves forward. With enough front brake applied, the rear wheel will start to lift off the ground. The rear brake does not contribute to braking at that point.

On a freewheel bike, the rear brake is neccessary mainly as a backup to the front brake since a front brake failure would leave you with no brakes at all.