Behold, the Man

John's gospel is unique. What makes it unique is the author, John, and the perspective he brings to the person and character of Jesus. John was Jesus's closest friend. You might say his best friend!
Peter, James, and John were part of an inner circle that Jesus included exclusively. Only these three were allowed into the room when Jesus raised the synagogue officials daughter from the dead. Only these three were invited onto the mountain to witness his transfiguration experience. It was John that leaned on Jesus's breast at the Last Supper and was told who the betrayer was. And in Jesus's moment of deepest distress in the garden before his death, only these three were invited deep into the garden to pray with Jesus.
But more than all that, John was the first disciple to be chosen and the only one at the very end. Only John was at the foot of the cross comforting Jesus's mother while the rest hid in fear. And it was because of John's unique relationship with Jesus that Jesus committed the care of his mother to him.
John writes long after the other gospel letters had been distributed among the churches. John describes seven miracles, five unique to his gospel. John includes seven I am statements and is the only writer that uses the phrase Truly, truly to underscore the extreme importance of these statements.
John writes not as a follower, and not just as an eyewitness, but as Jesus's best friend. And John writes for one single, specific purpose: That you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name (John 20:31). To miss this is to miss the very reason for your existence on this planet!