If they can ensure most Apple devices have IPv6 connectivity, there are a bunch of device-to-device connectivity benefits. For example, they can do video calling without having to host expensive proxy/STUN/TURN servers. Peer-to-peer data distribution and storage starts to make sense. Peer to peer gaming stops requiring server infrastructure.

It also means that typically all the other devices in a users home will be able to get IPv6 - so they can for example continue to remote control your Apple TV even if you walk a little out of wifi range.

Sure - for any IPv6 feature they build, they will need IPv4/proxied fallback. But if they can make sure 90% of users use IPv6 and direct connections, then the money and user experience costs of having to run the fallback servers becomes manageable.