Because not all of us have economical intl roaming plans like your carrier. The cheapest intl plan ATT provides is 120MB for $30. That could suffice for me on a normal usage at home for a few days when I have access to my computer, and don't always need my phone for directions, maps, etc. But when I'm travelling everything is done on my phone: texting, skype/whatsapp, looking up directions, restaurants, and especially for Tokyo, maps. I can burn through 120MB easily in just one day. Once you go over, you get hit with the overage charges.