The second lobe offers better breathing to produce power above 5000ish RPMs. The first lobe runs out of wind at higher RPMs, but offers less emissions and better fuel economy. Most people spend more time on the first lobe, and thus, get better fuel economy.
Best not to think of it as a second cam, Its a second stage of the intake cam, as the guys are pointing out. If you go look at the Dyno plots of our engine you will see where it kicks in, extrapolate the two curves (Before and after 2nd stage) and you will clearly see why it doesn't stay active. The first stage doesn't allow the engine to breath at high revs. The second stage has too much lift and duration to make power lower in the rev range. So the 2 stage cam is actually a brilliant concept, you get low end drivability and then the high end performance like with racing cams in other cars.