Simply because this is what enhances the power.

In order to maximise the explosion within the combustion chamber area of the cylinder head, we need to let as much as possible air / fuel mixture into the chamber.

This mixture enters the chamber through the inlet port and is controlled by the inlet valve.

Once this mixture is in place and gets ignited by a properly timed spark, the piston is forced downwards.

The bigger the explosion, the more the downward force and the result is "More Power".

After the combustion stroke has taken place, the burnt gas exits the chamber past the exhaust valve and through the exhaust port into the exhaust system.

This entire process is enhanced when the cylinder head has been gasflowed.

Without going into great detail for obvious reasons, we use a flowbench on which we are able to measure port flow before and after the head has been flowed.

In most cases we are able to increase the flow through a port by roughly 20%.

This is achieved by carefully removing sharp edges and rounding acute corners, concentrating usually at the roof of the port where the highest flow activity takes place.

We remove as little material as possible to ensure a lower overall port volume.

Depending on the requirement, in most cases we tend to go for high flowing, small, high velocity port, which produce good top end power with virtually no bottom end loss.

A great deal of time is spent on the valves and valve seats, where we gain good air flow with small valve openings.

The combustion chamber as well as the exhaust ports are also changed considerably to promote more complete combustion.

You need to remember that there are many ways of increasing the power and torque of a vehicle - bolt on kits, tuning chips, airfilter changes and exhaust system modifications mention only a few. 

But, you need to understand that the big gain comes from a good combination of all of these bolt ons and tuning techniques packaged with a good cylinder head flow job. 

The cylinder head will always be the key component of a performance enhancement package on any normally aspirated engine.