- Cutting some segments of the video: say you are recording the screen, and during the session, you tab and there are some information you don't want to appear in the video at three different places. Cutting the three segments isn't trivial in ffmpeg.
- The above but keeping the audio and removing video in those segments, because someone is talking.
- The inverse of the above: keeping the video but removing the audio because there was a private call for example and you kept rolling.
- Concatenating videos with different codecs is not trivial. If you have 200+ videos you want to make into one, and there are some that have a different encoding, you get warnings and the output throws the player off.