We got the idea with the Hercules that you could resupply military fleets or save fuel with launch your cargo ship into the air if time became an issue(because of the war). It was misguided, but at least a reason for the amphibious design.
Why are they building giant amphibious cargo planes today? Who has that need?
There are these islands which are kind of contested and which China is trying to assert sovereignty over. They are really small and many won't have room for an airstrip.
I *guess* that being able to quickly resupply these islands from the air would be very handy for China.