Standard theaters don't need higher resolution in part because nobody likes sitting close enough to the screen (first few rows) that the resolution is readily noticeable, and outside of the large IMAX theaters, the average screen distance to seat ratio isn't big enough to justify the cost of higher resolution projectors.
Possible 8K Uses:
* Large, High Resolution Desktop monitors for artists, as well as for developers that like densely packed screens. I'd gladly trade my current multi-display setup for something like a 50" curved 8K display.
* 3D Movies at full resolution and frame rate
* Virtual Reality (even with fancy lens blurring at the edges, current resolutions don't come close to matching the 'resolution' of the human eye)
* Large (full field of view) TV screens at closer distances. Admittedly 4K is good enough for most standard TV sizes, but 8K becomes noticeable if you want to get up close with a 120"+ screens without seeing the pixels). Of course, home projectors at even the 4K resolution are still quite costly
* 8K in anything smaller than say 100" is likely useless, just as 4K is a waste in screens smaller than say 40" outside the desktop environment.