Purpose of CHT drop in piston engine shutdown procedures?
According to this document, this is to allow the engine to cool and level the oil temperature:


  CHT Drop- Wait two minutes or until you can clearly see the H on Cyl Hd
  Temp. This is to allow the engine to cool and level the oil
  temperature. Closing down too early may damage the engine.


This is most probably to prevent damage to the engine due to shock cooling. Any rapid decrease in temperature during shutdown can cause damage to cylinder heads However, note that this theory is quite controversial. This is discussed in detail here and here. Especially, as @Lnafziger points out in his answer,


  ... in fact the engine cools much more quickly when you shut it down at the end of a flight.