Well the car does the math for you, but it is meant for comparison when purchasing because range relies on fuel tank capacities. where as L per 100 k is a fuel efficiency rating regardless of tank size. But it it makes more sense when you have a fleet of vehicles and are doing logistics and need improvements. if you have X dollars to spend on economy it is a linear equation compared to convoluted deminishing returns on low mpg vehicle. Like the quote mentioned 5mpg improvement on one vehicle is not the same efficiency as mpg on another vehicle.