Before backing the tractor under a semitrailer, why does the AAMVA coupling procedure require the driver to check and, if necessary, adjust the trailer height?

AAMVA CDL Manual, Section 6.4.1, Step 6, p. 6-8

Getting trailer height wrong before backing under is a direct cause of coupling damage: too low and the tractor can crash into and damage the trailer nose, too high and the fifth wheel jaws may not close around the kingpin, creating a false coupling that can separate on the road.