On an 18-wheeler/semi-truck, it has practical applications to protect from a front-end collision with a large animal. "Bull bar" because on remote roads that I've witness (like Montana), there is "open range" for cattles (aka, free range cows). It can be a very dangerous encounter! Once, I was on a secondary road in Kentucky and enjoying the winding back roads. I came around a curve to meet a full grown Angus in standing the middle of the road. His buddies (2 other Angus) were on the side of the road enjoying the scene.
On a passenger vehicle, its primary purpose is a brush guard.