Why are lighters waved at rock concerts?

Why are lighters waved at rock concerts?
To show appreciation and approval. The crowd looks quite dark from up on stage, so the flames are waved to show the band that they are having a good time. The practice began during Bob Dylan and the Band's world tour in the early 70s. These shows were released as "Down in the Flood." The lighter waving is the cover of the album. Leonard Cohen asked the audience to hold up lighted matches at his 1970 Isle of Wight Concert as documented in the DVD of that concert.