Most engine builders are in agreement that one of the most efficient ways to improve the airflow volume and reduce turbulence through your engine is to optimize the shape of the intake and exhaust ports of your cylinder head through machining. In the early days of the horsepower wars, port shaping was done entirely by the steady hand of a machinist, and while this was the best method available at the time, it left a lot on the table in terms of accuracy and consistency — and ultimately horsepower.
In the decades since strictly hand porting performance cylinder heads was the norm, the world has experienced several technological booms that have brought welcome change to both manufacturing equipment and computer technology. Which eventually led to the two industries merging to create Computer Numerical Controlled (CNC) machinery.