This app is meant to do one dedicated conversion: from Litres/100km to MPG and back again. Note that it will NOT calculate fuel economy (there are other great apps in the store for that purpose). All it does is tell you the equivalent mpg figure where a vehicle uses a certain number of Litres per 100 km (and vice versa, of course).
Why is this useful? For anyone who goes shopping for a vehicle, or is out at a car show, and you
1) are told what the fuel economy ratings are in L/100km; and you
2) look at the salesperson and say "What is that in mpg?"; and you
3) get a look that implies you are some kind of dinosaur...
then this app will tell you the answer instantly - and you'll look cool and tech-savvy getting the answer.
Also useful, of course, for the salesperson who is getting the blank stares from the customer.
Simple, but effective.
Note that this app occasionally displays small banner advertisements.