[lmz]

Yes HK has its own stored value card, but you can't use Octopus in Japan, can you?

Also I'm sure you'd agree it's easier to trust something issued internationally if there was some element of online verification to it (admittedly probably not much of an issue for transit applications).

[harpratap]

This is where Apple and Google step in, you can load Octopus pay on your phone and start using it like a local. Apple even notifies you about Suica as soon as you land in Japan.