They repair a portion of the damage your vehicle has taken. There are other small boxes on the side of the road, which can either help or hurt you. Blue boxes will give your car a temporary shield, red boxes will let you gain back health by hitting other cars, and orange boxes will give you a turbo boost. What you do not want to hit are the gray boxes. They are TNT boxes and will inflict a serious amount of damage to your car.
White boxes are an enigma.
They can either add points to your score or take them away. I played it safe and avoided them. There are also Pitbull Medallions scattered across the tracks. My method on how to smash opposing vehicles was similar to how I would try to tackle an opposing running back in a football game. If you take too short of an angle, the car will pass you by.