You see (at least in the UK) dishwashers say they only use x litres of water per wash or even things like "this only costs 10p an hour running costs".
It feels like one less step to get to "how much would this car cost me for my 250 miles a week" - which seems more common than "I can afford exactly 2 gallons of petrol a week, how far can I get with this car".
Maybe I'd feel differently if we bought fuel in the same units we measured its consumption in!
It's when I see articles about space exploration with miles and pounds - it feels wrong.
Or if you know the price of gasoline, and want to know how far you can drive for some amount of money.
In fact I'm struggling to come up with practical circumstances where gallons per 100 miles is what I'd want to know, could you explain why that seems better to you?
mpg is used to indicate fuel economy, comparing among cars. People might think -- do think -- that the difference between a 15mpg car and a 25mpg car is similar to the difference between a 25mpg car and a 35mpg car. But the difference is nowhere close in amount of gas saved per year of similar driving between the two. The former is a great boon to the environment and the pocketbook, but the latter is much more modest.
That being said, it is tricky to move the adjustment between the numerator and denominator, and there are cases where this choice can be key -- "25 miles per gallon" is indeed a far worse metric to use than "4 gallons per 100 miles".