MIDI Sync allows an external device to control the tempo of your TC-Helicon unit. If you use an external clock source from a DAW or other device (Ableton, Logic, Pro-Tools etc.); you may want each device to follow the same tempo.
MIDI Sync allows this to happen. All of your related vocal effects, like Delay or Rhythmic, plus your loop controls (via LoopAssist) will be synched to the incoming MIDI Clock signal.
For more information on the technical aspects of MIDI Clock (Timecode), see: Wikipedia – MIDI Clock