Dog Moon
Adventurer
I'm surprised no-one has raised the reason which would actually make sense to use: Harmione is a mudblood, a person with wizardry powers born of mundane parents (parent?). Her racial similarity to other students is useful then to enhance the audience distaste for the discrimination that she faces. Placing a black actress next to an otherwise white cast would tie the discrimination to discrimination against people of color.
(The schools do reflect the regions from which their students are drawn. Compare the Hogwort's students with the students from the other schools. Students such as Cho are clearly the minority. And the inclusion of Cho as a person of Asian descent means that Rowlings was thinking of the races of the students.)
Thx!
TomB
Interesting. I hadn't even realized that. Maybe they mentioned it in the movies, but it's been such a long time since I've seen them I honestly can't even remember.