Between the prologue and the narrative is a section of introductory testimony (1:19-51).
An official delegation has been sent into the wilderness to question John. (Remember to keep straight the gospel writer and the wilderness-dweller, not called the Baptist in this particular gospel). John uses the words of Isaiah to claim that he is a witness to the Messiah.
Gail O'Day and Susan Hylen, in their commentary on John emphasize the importance of this concept to our understanding of this gospel:
To be a witness is to see something and to speak about what one has seen.... To be an effective witness in a trial, one must have seen something about which one then can give testimony. John has seen the truth about Jesus and tells about what he has seen.
Few of us have ever or will ever be put on trial because of our association with Christ. Yet, we have opportunities to be witnesses. What are we doing with these opportunities?
(If you're familiar with Boring & Craddock's New Testament Commentary, you will recognize their ideas in this posting.)