Although we're only reading certain portions from the Vulgate Cycle, it's obvious it's necessary because, between the several different writers, it fills in a lot of missing details/history. When reading Lancelot, the reader begins to appreciate the full view of how he was raised, how he came to be at Arthur's court, and, more importantly, how he came to be in love with Guinevere.
The Vulgate's Lancelot takes Chrétien's Knight of the Cart and not only fills in the blanks but also expands the story so that Cart makes more sense to a certain degree. It allows the reader to get to know Lancelot, fall in love with him, and root for him and Guinevere. Due to the authors' dislike of Arthur, it softens the adultery aspect of the story because the reader no longer finds Arthur as charming or as interesting.
Also, I'm slowly compiling a list of characters who need their own stories. Anna is chief among them; I feel like she deserves to be more than a sidenote in Arthur's story. Galehaut is another who I would love to read a novel on. His story is bittersweet, but it could be beautiful as well. I would also really like to read Galahad's story - how he grew up, what all he was told, etc. I think it would be an interesting insight into his character.