I guess it is to prevent water from going down that pipe when that feature is not turned on.
But why do I care. The whole reason I ask is when I turn the spa feature on, this thing rattles incessantly and is annoying.
If I take it out, and turn that spa water feature on/off, it works fine and is silent.
I am wondering if maybe it serves a more important purpose like preventing the hot tub from draining back into the pool when the system is off?
I really don't know. Any reason I shouldn't just chunk it and put the cap back on as is?