Doeas manual-aiming worth something? could i hit really the body part i've aimed for or the only difference from auto-aiming it's that i could hit a target different from the currently targeted one?

It only allows you to select another target than the one automatically selected when you use ranged weapons.

Can someone explain how this works?

I am playing with 360 gamepad.

with bombs and daggers, if you tap the throw button geralt just throws it at whoever is closest to the center of the screen. if you hold it down though you can aim where to throw it.

when locking the target, I noticed that if you have the ripost skill, a little icon appear showing that you can use ripost on the ennemy (while parrying), which I don't think happen if you don't lock it (never saw it).