What is timecode?
An absolute time reference in hours, minutes, seconds and frames, used to align audio with picture or with other machines. Distinct from bars and beats, which describe musical rather than clock position.
For song work, bars and beats are the more useful reference, which is why a musically-synced prompter follows position rather than clock time.
The moving marker showing the current position on a DAW timeline. Everything time-based in a session is measured against it, which is why a prompter that reads the playhead stays accurate through rewinds and loops.
Re-recording dialogue in a studio to replace or add to production sound, matched to picture. ADR sessions are prompt-driven by nature, since the performer is reading lines against a moving reference.