If you're a B-Spec fan, have driver two race when your driver one has a 'down arrow' next to his name will give him time to recover. I think the condition of the driver is related to race performance and experience gain. You can complete A-Spec and other events as well in order to change their condition. Drivers will also start to deteriorate in skill once the number of races start to pile up (I read around 400 in another post). Also as Jeremy1026 mentioned, they can be used for endurance races as well (can't confirm myself) but I suspect that there's a reason why B-Spec races have a required number of drivers.