What's wrong with ffmpeg crashing when you feed it invalid input? What's the alternative to it not crashing? Should it continue transcoding or should it exit quietly?
What's the problem here trying to be solved?
After finding a crash, a researcher will generally explore this to see if it allows for arbitrary code execution. If it does, it is possible for someone to create a weaponized exploit.
For a utility as widely-used as ffmpeg, any bugs pose a threat to many systems. Fuzzing it makes the Linux ecosystem safer for all of us (hopefully).