Old House - Backflow Valve Question

[Stinson]

My house was built in 1985. There is a back flow valve but if I close it off, it doesn't appear to shut off the autofill lines. Should I care?
I am trying to sort out the plumbing. We have a very minor leak and the autofill Iines were pressure tested and didn't find anything. Now I just want to be sure the back flow would cover the pool. Why would it be there for any other reason?

Images - three images of water line into house.
Fourth image are the the autofill lines. There are two turn off valves. One on the right turns off both pool and spa autofill. One on the left is a mystery and I don't know what that line goes too. If you look at the first picture right before the hose line there is a line that goes underground. I am not sure where this goes either.

Any thoughts or insights to this puzzle are welcomed. Thanks in advance.

 

[flynwill]

Your mystery backflow valve might be for irrigation lines somewhere, that should be fairly easy to check.
Is is possible your city has recycled water for irrigation? I ask because I see two 1-1/4" or 1-1/2" lines coming out of the ground, one goes directly into the house, the other seems to loop back into the ground after feeding the hose bib and backflow valve.

 

[Stinson]

Thanks @flynwill. I don't know what existed back then but we don't have recycled water today. Could it be that the one set is for all the outdoor lines possibly including the pool. I actually don't use our irrigation system since all the desert plants are mature enough now. I could try to see. I believe the shutoff in the second picture also shuts off the water into the house. Your observation does make me wonder about all this even more.