When President Kennedy was assassinated, the public was barraged with rumors, gossip, hearsay, and opinions. So the Warren Commission was appointed to investigate and report its findings. They produced a well-researched written record that answered key questions, putting to rest many of the wild rumors that were floating around at the time.
Rumors begin, and opinion becomes confused with fact.
That's why there's a need for a well-investigated, carefully documented report. The book of Acts is just that kind of document. The story about God's arrival on earth in the person of His Son and His life, death, and resurrection had been circulated by word of mouth for about 30 years. Thousands had believed these reports, responded to the good news of Christ, and found salvation. Early believers wanted everybody to know the good news.