The friends code does a bit more than that. I think its still not fully fleshed out on the Wii, but its based on the DS, where you can play online with each other with the code. With the Wii, you can also send messages, emails, pix... and share Miis.
I think the main thing right now with these codes is that your Miis can 'mingle' and go visit other people's Wiis. Eventually, you can have hundreds of Miis who just wander into your Mii plaza. But you have to have to have at least one friend before any Mii will visit your Wii, or vice versa.