Despite all the debates as to why the Gospels were written (ranging from conflicts in a so-called Matthean community, synagogue expulsions, identity formation, to counter the rise of Gnosticism, etc.), I still think one of the best reasons calling for the Gospels to be written was to tell the story of Jesus for a generation who did not know any of the eyewitnesses or have access to first-hand accounts of the Jesus tradition.
As Bob Gundry writes:
“Why were the Gospels written? A high estimate of historical authenticity deriving from the literary distinctives of the Gospels leads to a further, old answer. The spread of the church far from its place of origin and the dying off of Jesus’ original disciples created a felt need for records. Since Christians had not divorced theology from history, those records turned out to be both theological and historical. Theological concerns for the present prompted redactional activity and resulted in differences among the Gospels. Historical concerns for the past checked fantasizing and resulted in commonalities among the Gospels.” (Robert H. Gundry, “The Symbiosis of Theology and Genre Criticism of the Canonical Gospels,” in The Old is Better: New Testament Essays in Support of Tradition Interpretations [WUNT 178; Tübingen: Mohr Siebeck, 2005], 39).
Note also the words of Bauckham:
“In other words, the Gospels stepped into the role of the eyewitnesses, which they had vacated through death. They interacted with the oral tradition, influencing it, doubtless becoming partially oralized in the form of new oral traditions, but also functioning as the guarantor of the traditions, as the eyewitnesses had in their lifetimes, and as controls on the tradition, making it possible to check its faithfulness to the testimony of the eyewitnesses as now recording in writing.” (Richard Bauckham, Jesus and the Eyewitnesses, 309).
New Christ-followers in the years 70-100 wanted asked and wanted to know, “What was Jesus like, where did he go, what did he teach, why did he die, why he is still worth following.” I think the Gospels are, whatever their individual specificities, trying to answer those questions.
Have you read Robyn Faith Walsh's new book? Great piece of scholarship - but way to skeptical for me.