Understanding A-Roll and B-roll

 What is the difference between A and B roll?  Why is B-roll needed?

    A-Roll is the main narrative driver footage, it is the video or the audio that is telling the story. B-Roll is the footage that supports the A-Roll and the narrative it is trying to tell through more interesting visuals. B-Roll is needed to make your project more interesting, break up A-Roll, and  to help hide imperfections in the A-Roll footage. If you are just watching straight A-Roll footage it can get boring so having B-Roll to break up the footage helps make your project more interesting. It can help hide imperfections in A-Roll if you need to cut audio from the A-Roll footage but want it to look smooth you just cut to B-Roll with it to help hide the change.