Yes, some kind of confused. ;-)
Ok, let's assume that we have a turbine engine driven helicopter or plane.
a) turbine powered helicopter
I want to take off. I open the throttle to let's say 85 %, wait until all the nice engine gauges are ok. Then I change my collective until enough lift is created for takeoff. Now the helicopter takes off and as I am lazy and the engine is providing enough power at the 85% setting, I don't care about adjusting power to get a constant RPM (RPM should decrease due to increased drag, right?). Now I could in principle fly around without changing the power settings, right?
b) turbine powered plane
My nice little imaginery turbine powered plane is having a lever which changes the pitch of the blades. Now could I do the same as above?
c) something piston powered
I understand that in piston engines torque and RPMs are related in a certain way. So I cannot just open the throttles and only change the blade angles, because the engine wouldn't simply be able to output constant RPMs, right? So changing pitch does only provide an easy way for an efficient and machinery friendly engine operation, right?
Some additional thoughts about piston engines:
So wouldn't the ideal piston powered helicopter or plane not have a lever which simply gives us the lift we want to have? You move the throttle to "1 kN lift" and the engine computer looks into a table what RPM and torque is required and simply changes RPM and pitch/collective angle?