That's for winterizing. In summer you keep the valve on the extra hose turned off, and the valve to your fresh tank on. To winterize, you turn off the valve to the fresh tank, and turn the valve to the extra hose on. Then the hose gets put into a bottle of RV anti-freeze. You turn the pump on to push the anti-freeze though the plumbing system.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE