(John bore witness about him, and cried out, “This was he of whom I said, ‘He who comes after me ranks before me, because he was before me.’”) (John 1:15, ESV)At the time of the writing of this gospel, John the Baptist still had a following. John the Apostle, the author of this book, is addressing this following in several places throughout his writing. His concern was that John the Baptist's followers had missed what John himself had taught, that Jesus was truly the Messiah. That Jesus was what John the Baptist was all about.
This verse also reminds us again that Jesus did not find a beginning when He was born into this world. He is preexistent. John the Baptist connects this preexistence to the "rank" of Jesus. John was a forerunner to Jesus, yet the "after" one was actually the "before" one.