Ableton Link is designed to allow multiple musicians to jam together over a network by syncing the tempo of their projects wirelessly.
When multiple musicians are involved, starting playback without a one-bar count-in would leave them uncertain about when to begin.
The one-bar delay or count-in provides a clear cue, ensuring that all participants are prepared to start playing together in perfect sync.
Please think of the one-bar count in as if it were the drummer hitting their sticks together before the band start playing.