While the appearance of your grilles is of course critical to customer satisfaction, each grill also plays a very important role in cooling your vehicle's engine. If air cannot flow into the engine compartment, the car is at significant risk of overheating. While many modern cars can get adequate air cooling through vents under the vehicle, it would be more efficient and cooler to have a grille at the front.
So while the grille is important, that leaves a lot of leeway for how it actually looks. Just looking at grille designs through the ages, it's easy to see how grilles can define a car's style.
While grilles come in all shapes and sizes, some techniques for using them can even improve performance. Many modern cars feature aerodynamic technology that closes some of the grille grilles when the vehicle is traveling at top speed. This reduces drag and improves aerodynamic flow. Other vehicles have specialized airflow channels built into the hood so that air can easily pass through the engine and out, allowing the vehicle to maintain a tight grip on the track and effectively cool the radiator.