Such planes can also be very useful for disaster relief. When a typhoon/hurricane tears through island communities, often there is significant damage to infrastructure, and it may take days or weeks to get airports back up and running, if the islands had airports to start with.
Such seaplanes just need the weather to calm a little and they can land to offload supplies and/or rescue teams and evacuate casualties.
Such large seaplanes will also be immensely useful for large scale SAR, like if a large ship with a lot of crews and/or passengers goes down.
Such seaplanes can fly much further and faster than helicopters, and once on scene, they can land and either take on all survivors, or act as a mobile support base to help keep all the survivors alive until more help arrives to take everyone if there are too many survivors for the seaplane to carry.
Planes like these can save countless lives in the Mediterranean where so many overloaded migrant ships/boats had gone down.
Sadly, that is just an academic capability, as not only would politics make a sale to those European countries nearly impassible, I also don’t see any one of those European governments caring enough about refugee lives to spend hundreds of millions on a fleet of such seaplanes.
But those are all secondary functions. I see the primary function for these seaplanes as to help logistics in the SCS.
All the big islands only have a single airport with limited runways, so having such seaplanes that can land in the harbour and offload/load passengers and cargo at the peer will be immensely useful to help easy the burden on the airports.
It also means the PLA can airlift on men, supplies and heavy equipment even if the runways where damaged.
These seaplanes will also be able to service all the Chinese held islands that do not have an airport.