The logic is that it’s much faster which is important for code that runs on a large portion of the world’s devices. Pretty much anything to do with video is using ffmpeg. From a set top box, to your phone, computer, YouTube & Netflix, even on Mars.
Video processing is hard, and when you’re processing that much data a x10 speedup is huge. That’s why it’s written in assembly. And there’s really no downsides to it because the original implementation is in C (cross-platform), then there are handmade assembly versions for each specific platform (performance). Win-win.