Install an overflow on a 1000 liter water tank

Complete Guide to Water Tank Installation: Step-by-Step Tutorial and Pro Tips

In the image below a water tank is shown. I just can not figure out the purpose of that extra piece of pipe going above the tank. What is it for?

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500 Gallon Preconfigured Above Ground Rainwater Collection System

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Water Tank 1000L

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EVIL ENERGY Radiator Coolant Overflow Tank Universal Aluminum 1L – EVILENERGY