For a character this important, who meant so much to Carrie Fisher, it wouldn’t feel right for her to just be written out of the film and have her die or disappear offscreen without getting to see her.
Acting opposite the actress’ footage and not Carrie Fisher herself sounds like a bittersweet and strange thing, but Oscar Isaac felt that Princess Leia deserved a proper sendoff and he seems to be among the many that are happy that Lucasfilm found a way to continue the character’s story.