The added stability provided by the wheelie bar can allow some RCs to pulls both bigger and longer wheelies. That is, it can raise the front end up higher off the ground without flipping over and go a further distance on just the rear tires.
A wheelie bar can also be used to prevent wheelies. At high speeds, sudden acceleration can cause the front end to lift and without the front wheels on the ground the driver is unable to steer the vehicle. A wheelie bar, at the right angle, can help to keep the front end from lifting too high -- suppressing the wheelie -- and allow the driver to maintain better control of the vehicle.