Understanding FLV (Flash Video)
FLV was the container format used by Adobe Flash Player to deliver video over the internet. It was designed for a different era, prioritizing low bandwidth over quality.
Since the deprecation of Flash, FLV has become a "zombie format." The video data is still inside, but modern software doesn't know how to render it properly.
MP4 is the universal replacement. By converting FLV to MP4, you are essentially saving the video from digital obsolescence.