Why do they use the clapperboard?

Answer 1

To make it faster, when editing the movie, to organize and align multiple film clips. The information provided on the clapperboard will tell the editor, right at the start of the clip, when and where the scene was filmed and what part of the overall script it fits into. Usually, the editing software will select the first frame of the clip as a thumbnail to represent it making it super easy to browse the often massive library of files that make up a production. This is why the 2nd Assistant Camera (or Clapper Loader) is responsible to have the clapperboard updated and in front of the camera before film begins to roll. The sharp and brief sound of the clapper slamming down will be recorded by whatever studio or field equipment the production staff is using for the scene. When lining up the audio and video in post-production, this sound can be perfectly aligned with the visual frame of the moment of impact on the clapperboard, which will align all the recorded sounds and voice tracks for the rest of the clip.