As yet there are no details on why Apple has made it so you can only stream the 4K movies and not download them to your device.
So, who might want an Apple TV 4K?
iPhone X is still over a month away from releasing, but when Apple's next flagship smartphone does launch, it will have High Dynamic Range (HDR) content from Netflix ready to stream. However, it doesn't now support 4K video from YouTube. On sale today, the $179 media streaming puck supports 4K Ultra HD resolution for movies and TV shows. YouTube uses the VP9 video format for its 4K videos, which the Apple TV doesn't support. As this Apple TV 4K support document (discovered by MacRumors) explains, you will not be able to download 4K content directly to your device for offline viewing. The upgraded streaming box comes with CPU with performance twice as fast and graphics four times faster.
You've got yourself a fancy 4K TV, now it's time to get some 4K content on that bad boy. The Apple TV remote received a subtle but useful redesign that added a textured ring to the menu button. For connections with slower speeds, Apple will downsize the quality of the content. "An "Apple person" with a nice TV and a yen for improved image quality". If you have more than one Apple TV, for example, the new "One Home Screen" feature in the latest operating system lets you sync the home screen across multiple set-top boxes. But if you're planning to upgrade your television in the future, you could spend your money now to know you'll have something to watch on your future new set.