The only reason to turn the AP “on” when on he ground is to carry out an AP test.
Prior to SU12 (11?) this was a problem, because if the AP was turned on when on the ground, it could run into limit, and then would not “reset” when turned off, so when turned back on again in the air, the AP was maxed out, and would drive the plane into uncontrolled flight.
This was corrected in SU122 (11?) with the introduction of an AP mode option, where that limit was reset when the AP was cycled — BUT – then it was up to the aircraft Devs to set that correct mode of operation in their AP code.
Now, when this mode is set correctly, it is quite possible to do an AP test on the ground (ie turn the AP on, do the test, and then turn it off), and be assured that when it is then later turned it back on in the air, it does not immediately drive the AP servos into limit, resulting in very undesired control of the plane by a bottomed out AP.
THANK YOU Asobo, for addressing this and adding those AP Mode option 