I personnaly use it to prevent down the track that a rider is comming. Photographers, spectators, and others can know when a rider will pass with that. Also, with several whistles you can inform the rider that a danger is in front of him, and to stop him.
Course marshals whistle to alert spectators and other riders (who may be stopped, looking at a feature, resting, etc) that a rider is coming through on course.
Looked like a right effort to get down it, and no real crowd pleasing gaps or silly high speed turns. Not your normal champs track, but it'll be interesting to see who does well, especially with the conditions.