Balancing is stabilizing the masses. It isn't the true up as they call it at the mechanic's. Balancing is adding ballast mass to the circumference of the rotor with which we equalize centrifugal force.
Why balancing at the spot?
Doing such a balancing we don't have to dismantle the rotors and take them to the balancing machine, what for sure means lower costs. That way we remove inaccuracy which could be caused at the reinstallation of the rotor. In addition rotor is balanced at the working revolution which is very important if rotor operates near critical revolution.