If your're talking in terms of bonus tracks on albums (which is the same for all artists who release them in Japan), I thought it was something to do with economic balance, eg. compensating prices of CDs by adding extra tracks aswell.

The above point is valid also, but not in Bon Jovi's case I don't think. Def leppard is a band I can think of at the moment. I think on their Euphoria album the Australian version had 3 bonus tracks, because Leppard hadn't toured in Australia for years.

Someone else would be better off explaining it than me, mind.