Thust levers are driven by a little electric motor that is connected to the levers via a friction clutch. The clutch gives enough friction so that the motor is able to position the thrust levers when they are "free to move" but the friction is so low that the pilot can easily override the motor driven position manually.
THR HOLD means that the clutch is disengaged, so that faulty behaviour of the motor cannot be transferred to the thrust levers in a critical phase of the take-off and (as stated by Capt Claret) it gives the pilot the option to change thrust lever position without having to fight the motor.
Should the A/T fail to go into THR HOLD mode during T/O, then nothing is amiss as long as the motor does not display quirky behaviour. It may be desired to disengage A/T, just to be sure.