Why do Japanese versions of the same album often include an extra track or two not offered in other countries?

Answer 1

There's no real concrete reason, its much like sometimes there are bonus tracks in albums that are released in the US but nowhere else, same with Europe and Australia. Its really up to the artist if they want it there or not. So its not only for Japanese albums but for all of them.

Answer 2

It's a plus, no reason I would say that if it is to survey such extra songs, if they become succesful they might re-release that same edition in other countries including US if it's an american artist