The original comment was about comparing fuel efficiencies.
But to your point: mpg makes it easier to know how far you can travel on a given volume of fuel, L/100 makes it easier to compare fuel efficiencies, and to know how much fuel you would need for a given distance to travel.
To me the latter seems generally much more useful than the former. The former provides an easier answer to "how far can I go burning this tank", which feels like a much rarer use-case than "how often will I need to refuel to my destination" (and "can I reach my destination on a single tank") for the general population.
The best way I've heard it phrased is that europeans 2ant to know much much fuel they need to get to their destination, while americans want to know how far they can go on a tank. Its obviously oversimplified/not relevant/other pedantic nitpicking but it captures the different relationship americans in general have with driving and their cars
Maybe this is because most people with cars in America (especially in the 1950's) were not worried much about the price of gas. Europe had much higher gas prices and was much poorer, so how much it would cost (how much gas would be used to get somewhere) was more important for planning than knowing when they would need to fill up again.
With volume per distance, you just need to divide the fuel tank capacity by the volume and then multiply it by the distance to get that. With distance per volume, you have to multiply the distance by the total volume of the fuel tank. The first calculation isn't that much harder than the second.
Realistically though, once you've driven the vehicle for a while, you pretty much know the total distance it will get in a tank regardless of the way fuel usage is measured.
But to your point: mpg makes it easier to know how far you can travel on a given volume of fuel, L/100 makes it easier to compare fuel efficiencies, and to know how much fuel you would need for a given distance to travel.
To me the latter seems generally much more useful than the former. The former provides an easier answer to "how far can I go burning this tank", which feels like a much rarer use-case than "how often will I need to refuel to my destination" (and "can I reach my destination on a single tank") for the general population.