I guess Kwangtung has the advantage of being further away from aggressive Russians - unless they come through China. It’s also closer to India if that is your ultimate objective. Plus, the supply line to your factory in Japan is shorter than it is to, say, Burma.
Transports stationed off the coast of Kwangtung can travel to Japan, pick up troops, move back and unload all in one turn. Or, use their base in Kwangtung to amphibously assault India - Kwangtung opens up options. But you have to protect those transports AND protect your home waters around Japan against invasion. In some games though, the threat of invasion doesn’t exist.
Kwangtung extends your reach without overextending your navy - like Burma might. I still prefer Manchuria because it’s closer to Moscow.