If I recall correctly, it's to allow overflow of Sulfuric acid or gasses to prevent the battery from exploding which I have seen but not in 30 years. Be advised, many batteries look sealed but actually have serviceable cells, I know the Interstate my 986 had was accessible as many are. If you allow the fluid level in a battery that can be serviced to go below a lead plate, nine times out of ten it's toast...
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