I think it allows the charcoal to continue to smolder, at least to a small degree.
Very few grills are completely air tight.
Thats right it does a little bit.
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The burner tube stays wide open even after the ash clogs the gap around the burner tube. The make up air holes allow that to happen, the ones right next the hose connection to the burner.
Even if the burner is removed and the hole never clogs with ash the fire will go out, unless there's a leak in the lid vent or around the lid. The air has to be able to circulate through the kettle for the fire to live, if it dont circulate eventually the O2 gets burnt up and the fire goes out. I'v left the bottom vents wide open plenty of times and came out a while later to a cold kettle, although it may take longer it'll be cold.