It's really not. It's so reviewers will be able to say "it costs more than the last version, but you also get much more storage" and the average consumer is happy. If there were a practical reason for it Apple would put it front and center.
They also bought RAC9, developers of Sneaky Sasquatch (one of the best games on the service, in my opinion), and they've had an aTV version since day one.
In general, the idea of a single game being able to hop between a phone (which can turn into a handheld with a controller grip), TV box, and desktop/laptop is really interesting. Streaming can do that, but latency and bandwidth caps become issues; this sidesteps it by letting you run everything locally.