I imagine this might also be handy for next-gen TV displays... Update: also see Apple not unveiling new or updated external display at WWDC, announcements will focus on software (same source)
"As Apple wants to keep making MacBooks as thin and light as possible, which typically cannot include powerful GPUs by nature, the company has been exploring ways to introduce a 5K display that can be relevant with a wider Mac lineup — it wants to be able to release thin and light MacBooks and MacBook Pros that all work with its 5K external display.Apple readying new external 5K Display as current model goes out of stock, may feature integrated GPU | 9to5Mac
Apparently, Apple has settled on a solution: to include a powerful GPU inside the display itself, rather than relying solely on the graphics power of the connected Mac.
Technology within updates to OS X (MacOS 10.12?) software will enable the process to happen seamlessly and automatically. Upon connection to the new Apple Display, the Mac will intelligently decide whether to use its own internal graphics power or rely on the external GPU included with the Thunderbolt display; the more powerful GPU will be used while the less powerful GPU will be inactive. From what we understand, the GPU will not be able to work in concert with the internal GPU at all — it’s one or the other."