To cool the turbo for one
A while ago someone posted the results of some research on here. They fitted a thermometer to the turbo and looked at the temperature profile.
They found that the turbo was actually coolest just after touchdown. After all, on final approach the engine was hardly producing any power but was cooled by a significant slipstream. The turbo subsequently heated up while taxiiing, even at idle or near-idle power.
So "cooling down" the turbo at idle power might actually be a myth.
In any case, the interpretation of the two-minute cooling down period at my club (where we fly with Thielert 1.7 and 2.0s) is understood to include taxi time, as long as during taxi you don't exceed 1400 rpm. Since we generally need more than two minutes taxi time from the runway to the club, no further cooling down period is needed. (So far I don't think we've had any turbo issues.)
Nevertheless, if your POH prescribes a cooling down period you need to do so (I'm not going to tell you to disregard the POH or some other authoritative document), but you obviously do that cooling down not in its final resting place, do you? After the cooling down period you move the aircraft forward (a whole 10cm will do) to its "final resting place after the flight" in the meaning of the ANO. (Same for engine start: After starting, roll the aircraft forward 10cm so that it counts as "moves under its own power for the purpose of taking off" as far as the ANO is concerned.)
