It’s called a clapperboard, and if you were under the impression it was just for editing convenience, you’re only partially right. If you’ve ever watched a behind the scenes video or have seen an old Hollywood documentary, it’s probably something you’re familiar with.
Clapperboards are used in a variety of video production primarily for the synchronizing of picture and sound. As referenced above, they also are useful in marking particular scenes and takes to easier distinguish them from one another, which is greatly helpful during editing.
The reason they always clap is so that the synchronization can be made easier. All it takes to match the audio and video is linking up the clap sound with the clap image. That way synchronization doesn’t have to wait for a certain sound or for an actor to say a line.