Damnit, I just wrote a, to my mind very elegant, and very long, response to this, but here are the cliff's notes:
Star wars prequels are the story of Anikan Skywalker. We already know his fate, ho hum.
Spartacus, Gods of the Arena is the story of Gannicus, a new and original character, who's fate we don't yet know. Yes, it sets up elements to be in place for Blood and sand, but the heart of the story is about Gannicus.
Godfather 2 is a sequel, with prequel elements, and is primarily the story of Michael. We get flash backs of Vito's rise to power, which contrasts and contextualizes' Michael's story. It's success is in large part due to the skill of Coppula as a film maker.
My argument is about functionality, telling a new story about a new character, who's fate is unknown is not a prequel.
All of this again has nothing to do with continuing to enjoying a film you have already seen on a rewatch. It's apples and oranges. My argument is about the enjoyment of new stories about new characters which may or may not be set in a familiar universe. That is why Riley Scott was very specific about Prometheus not strickly being an Alien prequel.
I think a lot of stories are labeled as prequels for brand recognition, or clarity reasons.