If your car is stuffed full of books, spare parts, half of your wardrobe, year old junk food, tigers and random passengers, you’ll be carrying weight, which is having a negative impact on your car.
Each time you turn a corner, brake and accelerate, your car is having to work significantly harder to do what you want it to do.
Your suspension and brakes will also be under more pressure, while the fuel economy of your car will be significantly reduced.