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I believe it's a form of vent pipe to assist water in flowing in a pipe underground. Think like the vent pipe on the roof of your house to vent the sewer pipe to aid water flow (sewage) down hill in a closed pipe. The vent is up high so water will almost never back up and come out, but air can always get in behind the flow. You can move more cu.ft./sec. if suction created on a downhill run is vented every so often. But know, you don't vent when you pump uphill under pressure.
Think of it like a diffuser on the back of a race car. Cars go faster when the vacuum they leave in their wake is mitigated.
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