Yeah, it can feel like a waste of time, but then again, it’s also a few extra credits that could be handy if you needed a car for an event perhaps later on when thought about it... I dunno, GT3 got quite repetitive for me compared to the rest of the series if anything though...But my answer to the question is those races where you win the corresponding car by winning the races its supposed be raced on (for example Beetle Cup, Clio Trophy, ... ummm... the Pagani Races maybe, not sure though).