the earthing kit is designed to earth all the major points on the car chassis and engine, the idea of them are to bring down the electrical resistance between any body parts welded/bolted together.
On a new car i don't think you will see any benefits, however on an older car where corrosion/rust takes its place on the chassis the resitance becomes higher back to the negative on battery
So the electrical components don't work as effeiciently as possibly, some benefits I have personally noticed are headlights are a bit brighter and also I now get 2-3 mpg more with normal driving.
I made my own for around £20 so a worth while mod for the money, I only put it on as I like the look of the kit and I was not expecting any benifits but I did notice the above
HTH
Anton