I and my wife have used this many times when traveling in Japan. It is useful for the shopping suitcase, i.e. the big check-in one that collects the souvenirs, cosmetics, food/wine, electronics, etc. that are going home with you. It only needs to arrive before you depart a city, so that you can add some items and send it along.
Even if you don't buy stuff when traveling, if you would want certain gear for some legs but not others, you can use it as a locker, i.e. moving the hiking boots into your carry-on before the Mt. Fuji leg, putting the heavy winter clothing in after you leave Hokkaido, etc.
And in contrast to luggage handling in the U.S., Japan's luggage handling is quite cheap, usually on-time, and always handled delicately with very little risk of loss or theft.
Even if you don't buy stuff when traveling, if you would want certain gear for some legs but not others, you can use it as a locker, i.e. moving the hiking boots into your carry-on before the Mt. Fuji leg, putting the heavy winter clothing in after you leave Hokkaido, etc.
And in contrast to luggage handling in the U.S., Japan's luggage handling is quite cheap, usually on-time, and always handled delicately with very little risk of loss or theft.