Because there are countries on this planet where a gallon of gasoline may not contain much gasoline. It may be punched with vodka, alcohol, water, rust, vegetable oil or other chemicals they are trying to get rid of.
In the USA or Canada, or western Europe you won't have much of a problem. But when you get over into the former Soviet countries in Eastern Europe, gas is no longer pure gasoline (or petrol for the UK guys).
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