The Siege of Antioch had many situations in which the First crusade could have been destroyed. What if they had been? Would there have been other Crusades later on? What would happen to the Pope who ordered the Crusade?
The idea of crusading was actually quite a novel one at the time, and the concept's taking off to such an extent during the Middle Ages was largely due to the astonishing success of the First Crusade, which seemed to put a divine imprimatur on the crusading ideal. If the First Crusade was defeated before it reached Jerusalem, that would probably be seen as a sign of God's displeasure, and there most likely wouldn't be any more crusades for the foreseeable future.