You get to drive the majority of the cars in the game in Online mode saving you cash and it means you only have to buy a handful of vehicles in single player. This is also a great incentive to play online, just pick a new car every new race you enter as they are all fairly similar performance wise.
Also, it's extremely beneficial to do all of the world tour events as soon as they become available as they reward you with the best cars in each class. This also applies to the mcrae special event as it rewards you with the best rally car.
This then makes all future events a complete breeze and also helps you if you need to finish up any online races.
I'm also pretty sure that each event has a holographic number stamped on it which tells you your highest finish so you can tell quite easily if you haven't wont it yet.