Bill
Generally speaking exhaust gas flow and speed is why headers are used. Normally the exhaust manifold is built close to the engine with a sharp turn as the first thing. Sometimes the manifold even necks the exhaust down. A header smooths the turns out.
Since all an engine is is an air pump we try and remove as many restrictions as possible. Ahh but wait the plot thickens we have exhaust gas scavenging the slant 6 is Great at scavenging. Sooo we split he exhaust in two and bring it back to scavenge some more power from other cylinders.
This is just an elementary lesson if you need more let me know.
Frank