No need to close it. Welcome to GTPlanet, Vietorius.
The Sport and Semi-race items can be used to simulate different classes of production car racing, where rules might make some modifications illegal or limit how far you can change settings. These rules aren't in the game, but I'm sure that PD meant for people to use these parts for their own satisfaction.
The are also very useful in the beginning of the game, where you have little money. In fact, I hope money is harder to win in GT4, to make you choose upgrades more carefully.