Basically the kit plugs into the cars computer which allows you see faults with the car (not every fault, but you get the idea) which is what the use at the car garages.
You can also use it to run performance tests on your car, but most importantly to change the ECU configuration so that you car will run faster/smoother or whatever really.
Ever heard of or seen a superchip? It's basically one of those, but without actually putting a chip in, just changing your current one to make your car perform better.
The beauty with this is you could use the standard remap, then do quite a few things to your car and get a custom one made for you car with your new mods by sending performance results over to henk (of course that costs) and you can also pay for a new license and remap other clios.
Hope that explains it all.