Insurance is king !
I was asked to taxi an aircraft for compass swinging by an engineering company, dispite the fact that I was rated on the aircraft ( on my ATPL ) it was decided to issue me with am an engineering approval ( based on my part 66 licence) to taxi the aircraft.
The thinking being this was the company's insurance was based on its engineering functions and issuing me with an engineering approval would keep the insurance issues well and truly within the engineering sphere of the company's insurance policy.