As we know, when travelling, the amount of stuff you ultimately take home can increase, especially if you make friends along the way. Having an extra lightweight bag lets you throw some extra things in to this bag and carry it on to the plane.
On this particular trip, I was planning to ditch my worn out duffel after I delivered all the gifts I carried here to China. The things I carry home as gifts, hopefully will fit into the ultralight bag, making my trip home extra smooth, and without any baggage fees. We will see if I can actually come home with less stuff than I brought.