Why do the traveler have to pass through the fair to reach their destinsation

The travelers have to pass through Vanity Fair as it represents a place of temptation and basic human desires for material things like possessions, power and wealth. Turning away from temptation was a very important part of their journey to reach Celestial City, as it proves that the everlasting life they could achieve there is far more valuable that the things that this world can offer.