And if the check valve leaks the water can drain from the FDC via the automatic ball drip. Big deal in cold climates. Also if the caps on the FDC are tight and the check valve leaks the length of pipe past the check valve will be pressurized. The firefighter maybe injured by the cap when loosing it or taking it off.
From the Automatic Sprinkler System Handbook 2013 Ed.
8.17.2.6* Drainage. The piping between the check valve and the outside hose coupling shall be equipped with an approved automatic drip in areas subject to freezing.
Comment Section on the above.
In the event that the check valve in the Siamese connection leaks, the purpose of the automatic drip is to drain this water to a safe location and to maintain the piping between the check valve and the hose couplings free of water. Without the automatic drip, any collected water could freeze and prevent the fire department from pumping into the system under fire conditions. This automatic drip also facilitates any maintenance of the fire department connection piping, because this portion of the pipe is already free of water. Exhibit 8.62 shows an automatic drain valve used on the fire department connections. The drain should be located at the lowest point of the fire department connection piping to allow complete drainage.