Headers vs 340 manifolds

If log exhaust manifolds were as good as it gets, why did all the manufacturers, I can think of, - make later style performance exhaust manifolds with individual, directional exhaust runners ?
Surely not for any scavenging, less restriction, lack of room, cost, weight, - reasons ? ?

How the hell some of you spend 1/4 the price of a good header to get 5 - 10 h/p (lol) outta an "X" or "H" pipe absolutely astounds me ! !