Why was the gift of prophecy so important in the early churches?
The gift of prophecy—speaking forth God's Word—was crucial because the early apostolic churches had no written New Testament. The Gospels and Paul's letters had not yet gone out as the Word of God, so growing churches in the pagan world depended on gifted men to proclaim Pauline doctrine and God's truth orally.
Les explains that when Paul wrote to the Corinthians, the New Testament had not yet been written. The Gospels were not yet written, and Paul's own letters had not gone out as the Word of God. These early, primitive apostolic churches were growing and reaching out in the pagan world without the benefit of the written Word. So they had to depend on gifted men who could teach the Pauline doctrines, rather than what was still coming out of the Old Testament, or they would have had pandemonium. Paul, by inspiration, recognized that the most important thing the local church could have was men who could proclaim the truth of God's Word without having it in print. Les also notes that the gift would later fade away, because now we have the printed page and do not need men gifted to proclaim the Word without anything to go on.