The hottest lump of metal in your engine is probably around the exhaust port of one of the back cylinders. By having the CHT gauge(s) close to this point you can get a fair idea of how quickly you are heating or cooling the engine so avoiding distortion and cracking due to high temperatures and high rates of expansion/contraction.
By getting to 150C before shutting down you are limiting the risk of damaging the engine. Of course, its not the actual temperature that's important in this case but the difference between CHT and OAT.