The readme sounds angry. When you run this without ffmpeg, it prints:

    >python run.py
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.
    ffmpeg is not installed. Read the docs you idiot.

I found a use for face-swapping recently, when working with Stable Diffusion animations. The main challenge there is keeping the frames consistent between each other. Generating a face and then face-swapping it on the full animation helps with the consistency.