It's part of the Research & Development function of the game.
When you first start your career the car has had little development. As the season progresses R&D is carried out which can develop things like better fuel economy, better aero packages, and better brakes, things like that.
If I remember correctly, the number one driver helps decide how aggressively these changes are researched. More aggressive development = more likely that a new part may fail (but you get new parts more often). Usually the number one driver will get the new part first, then the number 2 driver (usually you are number 2 at the beginning). Out performing your team-mate will get you the number one slot
You also help in the development. When in free practice, when you select the Engineer, look for the option (it's not always there) to carry out an R&D task (rather than just going out on track). The task usually involves doing a few laps within a certain time, or doing 3 laps using full throttle for 80% of the time, that sort of thing. If you complete the task, then R&D (new parts) will come your way eventually.
One of the things that will come to you is the ability to change your brake callipers to small, medium , and large. Some tracks are better suited to certain sized brakes, it's all a question of balance of braking efficiency versus weight, versus heat. Brake pressure settings will become available too (at some time).
Cheers