Impact plenium

I would expect there are several reasons for moving the spring. 

1- A much smaller spring can be used if it is moved to the outside of the plenum under the valve adjuster knob. Smaller likely means less expensive.

2- Moving the spring, outboard of the plenum, leaves a bit more room for gas, inside the plenum, so a bit more power or a shot or two more.

3- If the spring can be accessed from the outside it makes it replaceable without tearing the gun all apart. So the gun can easily tuned with a valve spring change to change velocities drastically or just slightly as one chooses.