Curb weight is defined as the weight of a vehicle at the time of manufacture. This includes all the bits and pieces that make up a car, but not any of the bits and pieces you might add when you get in and drive. The weight of passengers and cargo is not included in the curb weight. Once you add in the extras like people and cargo, then you’re looking at the gross vehicle weight.
Weight reduction is essential for improving fuel economy and it’s why manufacturers strive to cut every possible ounce from new vehicles. This is one of the reasons they’re looking at new materials made from strange things like mushrooms and grass. Reducing curb weight is also important for performance.
Take two cars that are exactly the same, but find a way to reduce the weight of one, and the lighter car is going to be faster. That’s why enthusiasts lose their mind when they find out their favorite car is gaining weight from model year to model year. It’s the same reason they jump for joy when they hear that it’s lost a few pounds.