If HR passes me a stack of resumes then that's who I interview; if all the people HR passes me are white, then I'm left to either assume that these were all the qualified candidates who applied (or at least, to operate under that assumption).

If the process gets bounced back because the stack that was passed to me was filtered by HR's unconscious (or conscious) biases, that forces them to give me more diverse candidates to choose from; the best candidate may still be the middle class white dude, but ensuring that the hiring manager is presented with a broad range of options and not just Chad, Biff, and Troy helps the whole pipeline.