TBC's act as a heartbeat for video frames to prevent an analog glitch in the otherwise 30-frames a second of interleaved video. Those innocuous glitches can screw up edits, digitizing, and produce so many errors down stream when you make copies, etc. They'll sync different cameras to the same frame refresh so that live edits won't flip when you change camera views mid-frame. They are expensive and worth it if you mess with analog video.