Simple idea really.
Rather than be forced to train your driver on something before your next race, especially when you have everything (minus mental and technical) in the 19s and training the other attributes provides very little progression for the driver.
Instead being able to save them up to use when you really need it would be quite handy.
Ofcourse there would need to be restrictions to stop people saving them up and firing the driver and using it on the new driver.