What does Japan gain from this?
You see this being mutual currency swap ....japan will be able to trade with India using its own currency ...again able to save precious foreign exchange !
Besides Japan has been investing in several of India projects ....bilateral trade is likely to grow logarithmically in coming years ....
Japan's own economy( which is largely manufacturing based one ) is slowing down due to decreased exports ...
India is a large market ...with billion people ...No market based economy can ignore such large market in its own vicinity ...
India being Asian power siting over crucial ....Indian ocean region sea routes and traditional enemy to its own sworn enemy ...It is natural that in India , Japan sees an important ally ...
Given all these reasons it's natural ...that Japan has nothing to lose but lot to gain !
Prospective alliance with India is important hedge against aggression in South China sea ....
Recently China has been flexing its muscles in South China sea ....
But last thing China will want to to precipitate Anti China alliance in asia ...
Close ties with India might force China to tone down its military posture in South China sea !
With US gradually extricating itself from Asian quagmire ....It is natural that countries threatened by China will seek ...new allies to fend against menace of China.