The control track is what keeps the tape running at an appropriate speed through the VCR and keeps the picture stable on-screen. When editing, these tracks become altered. When the editing VTR is in assemble mode, it will record a new control track, audio tracks, and video track. It is very convenient to record in assemble mode, because each segment can be pieced together efficiently and quickly. This is not common in today's VCRs but was a problem when editing controllers were not available.
Many editors like to use insert, rather than assemble edits, to piece together programs.
Insert edits, unlike assemble edits, insert video and audio over an existing control track. This basically means that the tape already has video black recorded from a blackburst generator. With the stable control-track, insert edits can be made. Using the control track pulses on the videotape, the edit controller can track the number of frames on the tape. Load the tapes into the corresponding VCRs.