Rose adds to the story by putting another human face on what the First Order does to the galaxy.
The new extended canon has done an excellent job of detailing the devastation that the Empire and now the First Order has done in the galaxy by subjugating and destroying worlds, bleeding them of resources, exploiting their people, and discarding them when they’re finished.
Rose and Paige Tico bring that story to the cinematic universe and Rose specifically points it out as the disease beneath the opulence of Canto Bight as well as telling her own story. And it is significant that this story is told by a Vietnamese actor.
She is also the purest example we have so far of a truly ordinary person who becomes a hero.
Rey may be from humble origins, but she’s an insanely powerful Force sensitive. Finn has trained for combat his entire life and is a gifted strategist. Poe is the son of war heroes. She’s not a crack shot and ace pilot like Han. Not a princess, not a scion of a literal child of the Force.
She’s just a person who stepped up when it was time to step up and I love her.