I do HVAC and new to home inspection. On an AC and furnace install on a flip house I saw this. I turned off the valve and checked a hose bib and water flowed freely so it wasn’t some main shut off. What is this?
Sprinkler system and back flow preventer.
Yep…the vacuum breaker separation from potable water…
Vacuum breaker - isolates irrigation system from the potable water system.
Was it a smart idea for these valves to be open? I thought that too but I looked for the controls and couldn’t find them, nor evidence of heads in the yard. This is Colorado. That exposed line could easily freeze, expand and blow a hole it that hose line.
Clearly, it’s brand spanking new, so perhaps the old one froze 
Dom.
Can someone dummy down how this works. For one, should that line going in be exposed given this is Colorado? Me shutting off the valve right at the line coming out of the ground did nothing to availability to water at the hose bib ten feet away to the left of this. I don’t quite understand this thing.
Around here the valves are left at a 45 degree angle so one can tell that the sprinkler system has been blown out of water so it won’t freeze in the winter.
There are many online resources for backflow valves, vacuum breakers, and the general theory and/or installation.
Like Larry said at this time of year the system should be winterized if You did anything to introduce water to the system You need to let some one know so they can blow out the lines again.
I think what you are looking at is the outlet for water coming from the home and going out to the sprinkler system. During winter months the water should be turned off to the system from the inside and the sprinkler lines should be blown out. So in the winter there would be no water in the pipes you see or the ones underground.