What's the purpose of storage in AppleTV?

I just finished watching the keynote, and saw that Apple TV is available in two storage configs. I just realised I have no idea as to why one would need storage at all in Apple TV. It does not store anything, it only streams, either from Apple or from a local computer/iDevice. It's only the OS itself that might actually reside on the device. So, why any storage at all, and why two options?

Cheers

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