Your Costco membership is good in Japan!
As in America (or your home country), you can save quite a bit by buying in bulk. Perhaps more importantly, you can buy items that may otherwise be difficult to find in Japan. It’s also nice to stop by for a quick meal. Not that I don’t like Japanese food, but every now and then nothing tastes better than a 1/4 pound hot dog for 180 yen (fountain drink included).
Costco Japan website: http://www.costco.co.jp/p/?lang=en