I don't quite comprehend your question, but if you're asking about the advantage of Questbridge during RD, the answer is quite simple. If you make it as a finalist, and get rejected from the college match program colleges see that you were a finalist, and they want to broaden their diversity (cliche, I know) so they take students were finalists much easier.
Think of it this way. Last year, around 4000 people applied to Questbridge. 2000 were finalists. 300ish were matched to schools. And 700+ were accepted into RD.