What is the purpose of "dead-end" or dummy couplers on a tractor when no trailer is attached?

AAMVA CDL Manual, Section 6.2.5, p. 6-6

Leaving glad hands uncapped when no trailer is attached lets water and dirt get into the air system, which can cause valve problems or brake malfunctions later. Using dummy couplers, or locking the glad hands together if no dummies exist, is a simple habit the manual expects every driver to follow.