Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God.  (John 20:30-31)

John concludes the reason for presenting a new account, it is to provide signs that might convince that Jesus was divine.  A sign, the Greek is σημεῖον (semeion) points the way, and is an indicator but in the New Testament, sign synonymous with the performance of a miracle, a reversal of or prevalence over natural laws. John provides seven signs authenticate Jesus, turning water into wine (2:1-11) healing a nobleman’s son (4:46-54) and a paralytic man (5:1-15), feeding a multitude (6:1-15), walking on water (6:16-21), healing a man born blind (9:1-7) and lastly, the raising Lazarus from the tomb (11:1-44).  It could be argued that the greatest sign of all was Jesus own resurrection, but that was a work of the Father, the previous was those performed by the Son.

But why these, and why only seven?  As John says, and the synoptic gospels attest, Jesus did many, many others.  There must be an underlying reason for their selection.