Lip syncing

What is lip syncing?
Lip syncing is a technical term for matching mouth movements with spoken words. This technique can be used in any context, including live music performances, recordings and acting performances.

Why is it important?
Lip syncing is beneficial because it ensures that firstly the quality of the sound, as well as the overall performance is of prime quality. Additionally, it ensures that the artist will finish the song, if the case were that they had forgotten the lyrics.
The drawbacks of lip syncing, however, are firstly the controversial aspects of it form media and audiences. It is generally seen as a 'cheat', or a shortcut used by artists that do not want to actually sing at their concerts. This is seen as neglect and a lack of effort towards the audience at that time, ergo the general reason it is frowned upon. Similarly, it is seen as the artist being somewhat talentless, and almost unable to perform live in concert for the audience.
Another drawback is the room for potential errors. If one word is not correct, the music will still continue to play, and will be evident towards the audience that the artist is lip syncing. This will hinder the overall atmosphere of the show, due to the previously mentioned drawbacks listed above.

Notable occurrences in music
Milli Vanilli
 In 1990, during a live performance at Lake Compounce in Bristol, Connecticut, the recording of Milli Vanilli's song "Girl You Know It's True" jammed and began to skip, repeating the partial line "Girl, you know it's..." over and over. Due to rising public questions regarding the source of talent in the group, owner Frank Farian confessed to reporters on November 12, 1990, that Morvan and Pilatus did not actually sing on the records.