don't see it as a TV...
...but as a low-end iMac, ie screen with streaming and internet. Then it allows Apple a market segment below that of the imac. The existing Apple TV being a warm-up, an exercise- 'a hobby project' as Jobs called it.
Many people turn on their TVs in the morning, to catch the weather, traffic and general news. Siri integration would allow easy news searching, leaving messages for family members, review diary items. Typing is not a chore, but having a wireless keyboard floating around the living room / kitchen is. I say kitchen because this wont be everybody's primary set - not at 37 inches.
It's Job's 'Digital Hub' again.
So why this, and not just an Apple TV box plumbed into a Sony? Guess: The integration of a microphone, possibly camera for face time, and sheer neatness.