It's not really ideal for deadening road noise, for that you should use some thicker mat and put it on the floor pan.
Sound deadening is used to deaden the panels of metal to avoid ringing/vibration/resonance from the audio frequencies.
Ideally if you have upgraded to components at the front and a subwoofer setup, you should deaden the boot lid, and surrounding boot, along with the whole of the door. If you have the money, do the whole car.
It can also help to improve the SQ experianced within the vehicle, the frequency range and response, and improve the overall quality of audio for the people inside the car (and prevent people outside from hearing much of it).
There was a recent test of deadening products on a certain website (will try to find the link) and they found that Dynamat Extreme WAS NOT the best, even though it was the most expensive. There was a better product for less cost (again, the name escapes me). CAD (Car Audio Direct) will soon be selling this product in bulk packs, to reduce the cost even further!
I will be getting some when they do........