A few guesses:
1) to avoid stuttering from slow or choppy downloads. Lots of storage will help avoid this BUT: any meaningful size should be enough and the initial download will still be at the mercy of download quality (assuming the app or stream is used immediately)
2) to use when not connected to the internet. Maybe? But seems weird for an at-home device.
3) to store non-internet based items. Home movies? But 64GB (or even 32GB) seems like either too little or too much. Too little if you really want to use the Apple TV for storage. Too much if it is just a temporary repository.
None of these makes sense.
A crazy guess: for people on satellite internet?
Note that none of this explains why 32GB is the base amount (although perhaps, given the 20GB app limit, perhaps Apple figured the first 10 minutes of the most likely 25 movies to be streamed could be onboard for instant start? or something similar where the device tries to predict usage and have some portion onboard?).
A small bonus question: is the memory solid state? I assumed it was but, really, don't see why it needs to be given that the device will probably just sit on a shelf. And I don't think I've seen any confirmation of the storage type.