The use case definitely exists today:

  - 3D apps (games) render at a resolution less than the desktop for performance reasons (even in fullscreen mode to avoid switching the video signal)
  - Similarly, streaming sticks/TVs render the UI at 720-1080p and overlay on hardware decoded 4K video (very common in anything that's not a console, Apple TV, or NVIDIA Shield)
  - Handling non-high DPI apps on a high-DPI desktop, or handling systems with mixed DPI displays
  - Combining HDR and SDR content on the same desktop. Same with deep-color apps.
  - UI effects like the OS X genie effect, app thumbnails