It solves an issue of Japanese addressing system being a total mess. There is basically a wild wild west when it comes to the address part on most of the ecommerce sites in Japan: some offer address auto-complete via zip code, some don't; some require a building name, some don't; and the address itself may be written down in different ways. Having a source of truth in a form of a provider which has vested interest in keeping the address uniformly correct on entry is god sent here.