The Hub, as it tied the Wasteland together in Fallout 1, ties together NCR, dividing it in three. To the north, you have the political center of the State of Shady (along with Junktown). To the south is the industrial capital in the Boneyard, along with Dayglow. To the west is the presumptive agricultural center, the more-recently acquired Central Valley, with the state of Maxson, Sac-Town, and the Redding Territory (presumably also where most of the fighting of the NCR-Brotherhood War is).
The Hub itself, the economic and logistical center of NCR, presumably ties the country together with its roads and rail networks. If you take that, NCR is cut in three, cut off from each other and able to be taken piecemeal without coordination or supplies between the thirds. Once the Hub is taken I presume Caesar will want to go for Shady Sands first, take out the political center and break the will of the south and the valley to fight. The Legion moves up the valley itself, just a straight flat line without many natural defenses. Lightly settled with the Brotherhood pocketed throughout, the Central Valley will crumble pretty easily. It migth fall apart without the Legion even entering it, just from political chaos and the Brotherhood staging breakout raids from their bunkers and hidden outposts.
