Basically, it makes it easier to compare fuel economy differences at different points in the scale because they become subtraction problems instead of division problems. This is because usually the amount you are going to drive is fixed, and MPG puts the miles in the numerator, while "gpm" puts the fixed miles in the denominator, I think.
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.
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.