brings your north and south bridge together as a cohesive unit. basically, all the features of a motherboard arent ran off the same chip. theres 2. one handles the cpu and ram etc (i think) possible hard drives or something, while the other handles everything else (i think.) the concept is something like that, dont quote me on specifics.
you wont notice performance gains by having more bandwidth than you can use between the north and south bridge, however, nicer cpus and gpus and more peripherals and hard drives can saturate the bandwidth between the north and south bridge if the interconnect doesnt have enough bandwidth. this can cause a system to perform far under what it should. its like completely saturating anything with bandwidth, it becomes a bottleneck.
hyper transport is amds connection between the north and south bridge, if i recall, it has a lot of bandwidth.
sorry for the mess of this explanation, im pulling it off the top of my head