Besides pure looks, I think it's somewhat of a proxy for a set of culture/background elements that are hard to capture. There are explicit ones like "religion" and "interests", but then there's sort of a vague, "what sort of community did you grow up in?". Sometimes something other than ethnicity dominates there, e.g. if you grew up in a Mormon community, or a socialist commune, but for many people ethnicity, for better or worse, correlates pretty well with similarity of background/experiences.