The idea was that by repositioning the plug in the combustion chamber closer to the exhaust valve you get a more complete burn, Chevy engineers moved the location of the plug towards the exhaust valve to improve flame propagation. But, by moving the position of the spark plug in "straight" heads, similar gains were realized.
They are suppose to get you few more ponies, and believe it or not header clearance in some applications, go figure.
Cp