iTunes gift cards are perfect if you have a need to download games and media from the Japanese iTunes store.
Some music and apps are only available in the Japanese version of iTunes. For instance, I play a game called Jubeat Plus which is free to download from the Japanese iTunes Store, but to buy an in-app purchase for the game I used credit from a Japanese iTunes gift card.
Unless you live in Japan and have a bank card that is issued via a Japanese bank, then the only way to buy stuff off the Japanese iTunes store is to buy gift cards with a prepaid value on them, that you then redeem once logged in to iTunes.
If you don’t have a Japanese iTunes account and dont know how to create one, just do a quick search online where you will find lots of help with this.
To get hold of the iTunes gift card what I do is while I’m in Japan I normally stock up on the cards by buying them from a shop. Most big electrical shops sell them and you can also buy them in the discount store Don Quijote.
You can also buy the cards online, but normally there is some sort of extra fee involved in this.
Interested in playing Jubeat Plus? You can get it here: Jubeat Plus (Japanese iTunes)
Do you use the Japanese iTunes? What are your favorite apps?