IME it varies. Different 'style' bars are grown not just to reflect health status of hoof (which is a big topic of its own), but also the environment. For example it is not unusual for a hoof which is worked in soft conditions to grow a bar which is taller and straighter - which seems to give excellent grip. Hooves worked mostly on road/hard surfaces may have little or no visible bar. There is every permutation between.
Again IME unsympathetic trimming can undermine a hooves best efforts at adapting to its working environment. But that doesn't mean I am anti trimming or anti trimming bars. There are times when it is the very best thing to do, but you have to know why, what, and how much.
Their function is strength. At the bar, the white line actually turns the bend and goes around the corner into the bar. This gives each side of the hoof a curve of horn to stand on, and any look at architecture will tell you what a strong thing a curve is. Early bridges, before they invented suspension bridges, have arches. Nature knows the trick too. If you put a board on top of it, an egg will support the weight of a human.
Early barefoot gurus would tell you that in a perfect foot the bars should be straight. I've hardly ever seen a straight bar in any foot, they nearly all curve and I believe that curve, too, is to add strength.
A weak foot will grow longer bars and a really weak foot will grow a bar right up round the point of the frog in an unbroken line from heel to heel. That support should be removed inextremely rare cases - it will fall off when the foot is strong enough to do without it.
I am sitting here holding the piece which simply peeled off my latest rehab from around the point of his toe once the back half of his foot developed enough strength to do without it - this is it: