Not just on 73s but any high bypass engine:
To make sure the N1 spool is free to move and there is not a puddle of ice locking it, (first flight of the day), or some FOD.
A spinning spool allows you to see the stator vanes and the compressor intake behind the fan more easily.
Ditto to see the vanes in front of the turbine at the rear.
Ditto to see the condition of the rubbing seal strip round the edge of the fan.
The fin will have been cold soaked during the cruise and if the aircraft desends through moisture, ice may form. The base of the fin area might be aerodynamically 'quiet', so perhaps not enough airflow to disperse any ice, or more likely: the ice may form on the ground during the taxi in to the gate. A similar thing sometimes happens to the landing gear and the lower surface of the wing, under the the fuel tanks.
Last edited by Uplinker; 7th Jan 2017 at 14:28.
Reason: I think we crossed Iomapaseo