The stopping position is what drives every parking system (marshaller or docking) and decides where the plane should stop. Please read the manual, which explains the relationship with it, and the plane preferred exit. The goal is NOT having the airplane front gear always in the same place, because that's not what happens in real world, but rather have the preferred exit in a predictable position to have an easier time operating a jetway.
The main reason for wanting to edit the main parking position too, is that in some cases, if you had to move the stopping position of a considerable distance, it *might* happen that GSX won't detect if you are inside a parking, since the detection it's made from the center of the parking so, in such cases, it might be best to move the main position (of the parking) to have the plane in the same position as the edited stopping position.
But usually, the main reason for editing the parking itself, is to fix errors that lots of developers do, with the AFCAD parking not properly aligned to the yellow line so, it's usually more common to rotate a parking, rather than moving it.