I think that it's there so that you can play a joke on your Brother-In-Law...
(Make sure that it's closed...)
When you're getting ready to dump the tank: tell him to open it up when you yell for him to do so...
Now: you head on over to that second valve, and wait...
Yell for him to let 'er rip... Let him say something like "What the Hell: nothing is coming out!"
When he walks over to pick the end out of the sewer connection (somebody WILL do this...): give it about three more seconds, and then pull the lever!
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