The car didn't have a diff/lsd, so they would Jack one side just off the ground, run it up to 45 but due to one tire being stationary the tire spinning was doing 90. From there they would move the weight around until the vibrating essentially stopped. Then do it all over again for the other side. Therefore balancing the tires on the car.
The drivetrain wasn't balanced so the tires were used to balance the whole front end