The first rule says:
The Road is trodden in the full light of day, thrown upon the Path by Those Who know and lead. Naught can then be hidden, and at each turn, a man must face himself.
At every turn a disciple makes - in order to serve - he or she faces their own shadow cast by the light from those ahead on the Path. This shadow is the obstruction to the Light resulting from the imperfections in the disciple’s own service expression. Perhaps this shadow-casting is the reason why service is said to be the way of advancement. No clearer way of seeing one’s own faults could exist.