In particular to hide the fact that they have edited a bit out, as the missing bit is not obvious in the sound, but would be in the picture, which would jump a bit and the joining up of two bits would be obvious.
The original Channel 4 Friday Alternative news programme demonstrated it by something like filming a well-known politician entering a building going up some steps (the camera and the journalist both at the top of the steps) then after he had gone, the cameraperson goes to the bottom of the steps and films the journalist, stood at the top of the steps, saying something like "Here comes Mr. X" or "Good morning Mr. X".
Then when it appears on the television news it looks like the journalist said it at the time and may look like the politician did not respond when "Good morning" had been said to him even though it hadn't.