Tuning a pcp

If I am understanding you correctly, the adjustable power wheel is adjusting the transfer port.
I currently have the Hatsan Gladius long, which has a power adjuster. I have tuned down the hammer spring and left the power level at max. This gave me the best shot curve for the particular pellet that my gun likes. The pellet is a little heavy so if I decide to shoot a lighter pellet, all I have to do is turn down the power wheel and not mess with the hammer spring. The curve wouldn't look as good but for temporary use, its good enough. IMO, All tuning does is optimizes the gun to the pellet.
I think there are guns out there that don't require much adjustment but since they are so expensive, I have never had a chance to mess with them. Even with these, I would think that some adjustment is necessary to maximize that gun's potential.

My Texan let's me adjust (fine tune) the spring tension for lighter or heavier bullets.

I think the biggest benefit of being able to tune up or down is if your back yard target shooting you can turn the power down and get more shots then you can take that same gun turn it back up and go after critters also if your hunting those critters in a dairy or the like and find you are blowing through you can turn the gun down to insure your not punching holes in things other than the critter your hunting

For me, tuning is almost as much fun as shooting! It just amazes me that so much can be had from a little puff of air.
And to think, 3 years ago all I knew was what was sold at the big box stores and felt that this is all kids stuff. Boy was I wrong