AV sequences are a great way to present a series of images to your audience, setting your pictures to an accompanying soundtrack. An AV might interpret a song, or a poem or a piece of music through a series of pictures, or tell a story or document a theme by combining music and pictures.
Have to say, it is also a great way to ruin a favourite song(!), as you find yourself listening to the song so many times, in order to perfect the transitions between images.
Once Upon Another Time is a collection of childhood cine-camera clips, edited together and set to the sound of Sara Bareilles.