I use GPS1_POS_X and the one for Y and Z on my multicopters, but why?
I put the multicopter on the ground, soon to be HOME.
Then I fly. Then I land.

When is it useful?

And what happens if the GPS is long way back or long way up or to the side? And dont use the POS-parameters.

When I recently set up a RTK-multicopter, I put the position of the GPS centre relative to the cameras CCD-centre. That felt somehow more useful. Was it right?

The controller (IMU and GYROS) center is the reference for all the math calculations.
The controller expects the GPS antenna to be located very close to that place.
If not, use the PO_X, Y, Z offsets. the controller will then correct it’s maths to compute the correct values.

The camera also has a POS_X, Y, Z offset, again relative to the IMU and GYROS of the controller.

The camera offset parameters are not in master yet.
But you can try rebasing this PR https://github.com/ArduPilot/ardupilot/pull/6266 on topof master and compiling it from source to get what you need.