Officially, the point it's fuel economy, but I suspect most of the economy comes from the new 8-speed auto (also most of the weight reduction comes from it), the real reason is...cost.
I doubt that the N55 costs less. In another N55 related thread, I quoted a well respected engineer from the blog GreenCarCongress who referred to valvetronic as "hideously expensive" and "similar in complexity to a swiss watch." Whatever money BMW saves by going to a single twin scroll turbo and by using less material will surely be offset by the increased cost of the valvetrain.
The 8 speed auto helps with fuel economy, especially if it includes ZF's optional integral start/stop function. Valvetronic on its own is good for ~7% increase in fuel economy. BMW quotes the N55 as getting 10% better fuel economy compared to the N54. IMO, the N55 will get ~15% better real world fuel economy if it gets mated to the ZF 8spd auto.