Why should we read John's Gospel sociologically, that is, as a history of the Johannine community? Why not read it theologically, as an offering to the Christian communities of John's theological reflection, a reflection (no doubt) responding to various challenges (e.g. deteriorating relationships between synagogue and church), but one driven more by the essential question Jesus posed. Who is Jesus?
In this light, the prologue is very important. Whether or not it is adopted by John into his gospel (from another theological source, as a Christian hymn, even as an adapted statement from another religion), it sets out a theological focus in relation to the question, Who is Jesus?
Jesus is the Word made flesh, the only Son of the Father, the one who makes God known in the fullest way to the world.
" And the Word became flesh and lived among us, and we have seen his glory, the glory as of a father’s only son, full of grace and truth. (John testified to him and cried out, ‘This was he of whom I said, “He who comes after me ranks ahead of me because he was before me.” ’) From his fullness we have all received, grace upon grace. The law indeed was given through Moses; grace and truth came through Jesus Christ. No one has ever seen God. It is God the only Son, who is close to the Father’s heart, who has made him known. [NRSV]"
This christology is not disclosed (save implicitly) in the three Synoptic gospels, nor is it as explicit in the writings of Paul (though in various ways Colossians and Ephesians go very close). Need we pose any major cause for the writing of John's Gospel, any significant origin for his Gospel other than the reflection of an agile and questing theological mind as it posed then answered the question, Who is Jesus?
From the prologue, the ending of which is cited above, the Gospel of John flows readily as an account of who Jesus is: the Word become flesh (the Word which created the world now, in the flesh, transformed it through signs), with a glory seen by eye-witnesses, disclosing a message of God's love ('grace and truth') received by many (and rejected by some), in the course of which mission of transformation, God himself as Father identified with his Son, has been revealed in a manner not seen before.