While you are correct that in most transport jets the flight controls are powered (the Direct Cable 9 is one exception) there is some feedback to the pilots from the system. Pilots over time learn the "feel" of the aircraft and know how much force they must apply to raise the nose. Triming the plane keeps this feel constant so they do not have to think, "We are nose heavy today so I will have to pull more" or "We have an aft CG so I will not have to pull as much as normal to raise the nose."