[Jaydelart]

There can be a variety of reasons. Probably the most common reason would be to help enable them to live there -- as JayT said. I don't have much personal experience on the matter, but I've heard that Japan's work standards are no joke, and I have little reason to believe otherwise. Anyone that would consider getting involved in it should have a realistic idea of what to expect. And, I would doubt the majority of those who dream of it do.