In short, speed. G provides UP to 54 Mega Bits Per Second Mbps. 802.11 provides UP TO about 120 Mbps. I don't like the way they sell this because who has a connection that even comes close to 54 Mbps let alone 120Mbps.
The idea is that if you have more computers on the N network, it won't slow down as much. But still you have a garden hose feeding a sewer pipe.
Some users get better connections with N because the signal is sent out twice the rate as G. Still, I see no need until you get a least 20Mbps in your residence. Of course this is a subjective analysis.