Very common these days are remote filling points. They allow a convenient place to add water and coolant, and allow smaller radiator to be mounted in  untraditional ways and places, under panels, or structural bracing, to allow better air flow  (wind tunnel ) characteristics. Moving the radiator lower, or frontward allows smoother air flow over the vehicle. However, the radiator may no longer be the highest part of the cooling system, and can cause problems with trapped air. However, if the highest point in a cooling system does need  the capability to "burp" out the air and replace it with coolant. A remote filling point, or coolant reservoir (also called a degas bottle) fit that function perfectly. Many times it is much more conveniently located than the traditional radiator cap it replaced. Another function of this bottle is to allow expansion to occur, while keeping pressure under control. As a vehicle cools down, contraction also occurs, so the bottle must have sufficient quantity to store  enough coolant.