This app was mentioned in 1 comment, with an average of 2.00 upvotes
I don't think it's necessary. I've never gotten one in my 10+ trips to Japan. Just preparing yourself while you're online so you're ready offline is all you need to do. I usually save some Google Map data via cache and having a transit map (which is at every train station) is really all you need. There is plenty of wifi in Japan to survive too. Nearly every convenience store has some now and they're everywhere. I recommend having an offline map and downloading the following apps and you'll be fine.
https://play.google.com/store/apps/details?id=jp.ne.wi2.tjwifi