I believe it's not a check valve but a backflow preventer. If the house pressure is greater than the street pressure it vents.The expansion tank is for expansion of water pressure when the water is heating so the backflow preventer won't vent. The reason for the backflow preventer is that if the water main breaks and loses pressure all the homes would drain into the main possibly contaminating the main and leaving homes with air in the plumbing.I like the supply valves I don't use them I just turn the main off when doing any plumbing repairs.
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