I've often wished this feature actually, especially when submitting pull requests, and you'd like to showcase the new features. I've often had to either embed a video link or convert to gif.
As someone who likes to do previews in PRs, mp4's are infinitely better than gifs. Better colors, better frame rate, better resolution, better size when you change any of those factors. It's really, really silly GitHub still don't support them.
Actually, my use case is clicking on other people's GitHub links from time to time and not wanting 5 MB of download for 100 KB of data.
I have a video in a gitlab readme, it's quite nice to show how my otherwise quite academic genetic algorithm toy project looks when it's animated.