On the ATR, wiht PW120´s series engines, it is reccomended 1 minute minimum of cooling time, so the ITT will be for the next start at or below 200 ºC.. .. .On the taxi-in the tasks are for reconfiguring the systems for the next flight, and besides anti-ice, there is no need to shut down, hydraulic, pneumatic nor electrical, they shut down by themselves, actually yous just sit and watch!. .. .So the real reason is for cooling the engines, they are encased and they run hot (600ºC)