(From Visions of History: Robert de Boron and the English Arthurian Chroniclers by Edward Donald Kennedy)
The way I interpret the French Tradition is that it "started" with Robert de Boron. In a way, Robert de Boron is like the "Second Father of Arthuriana". Robert de Boron didn't so much alter King Arthur's story as he instead gave it a new context.
Despite Chretien de Troyes and Robert Wace introducing the Round Table, the Holy Grail and Sir Lancelot, it was Robert de Boron who truly established the "Christian Chivalric Universe" - by writing about the history of the grail and the introduction of Joseph of Arimathea as a patriarch, Robert de Boron gave the (Literary) Chivalric genre a Mythic Past and a continuity of sorts. It fits in the Christian worldview of history as part of a giant plan - and story - of God's for Mankind's salvation.
It fully establishes the Christian origin and heritage of (Literary-)Chivalry-as-an-institute by not only having Joseph bring Christianity to Britain, where the future fantastical setting of Arthur and his warriors was to be established, but also having him serve as the ancestor to multiple Arthurian characters such Percival, Tristan, Lancelot, King Lot, his son Gawain, and Arthur himself (fulfilling the aristocratic need for genealogical fantasy).
This essentially pushes away, and negates the symbolic consequences of, the original narrative set forth by Geoffrey of Monmouth - the almost kind-of secular and worldly portrayal of Arthurian History.
For all intents and purposes, Joseph of Arimathea replaces Brutus in importance for the Era of Camelot. This is why (in Vulgate cycle, at least) Joseph is said to be a knight.
Nowhere is that felt more than in Robert's introduction of one of the most iconic pieces of Arthuriana: The Sword in the Stone.
The Sword of the Stone, an item not mentioned at all in Historia Regum Britannia, and its attendant story arc of Arthur's hidden upbringing by Antor/Ector, essentially functions as a second origin story for King Arthur without having to negate or omit Geoffrey's older story of Arthur being born through a thoroughly un-Christian manner.
Whatever or whoever Arthur was before pulling the sword doesn't particularly matter, the sword in the stone effectively grants the same Divine privilege to rule as if he was conceived and raised as a normal royal.
AND
Arthur's ignorance of his heritage affords him synergy with his knights, many of whom are often of the "Fair Unknown" archetype, including Lancelot, Gawain, and of course, Percival.
