This does not provide any capability that these devices did not already have (smartphones already offer tethering and 3DS can connect to it like any other Wi-Fi hotspot). It only makes it easier to do a thing you could already do.
Nintendo needs this in Japan because fewer and fewer people bother subscribing to any home internet service at all (they don't see the point, when cell phones are so good). That means there is no Wi-Fi in the home. And free public Wi-Fi hotspots never really caught on in Japan like they did in the US. So that's a problem for Nintendo, if people have no way to connect to the eShop to buy stuff.
The point of this application is to make it easier to tether (again, a thing you could already do, if you knew how) to encourage people to do it and to make up for Wi-Fi becoming less common in Japan.