Proving that you were discriminated against is notoriously difficult. An exception to that is when the offending party is essentially
admitting guilt in their effort to reform.
Implementation of the Rooney Rule is an admission to guilt of failing to comply with EEO. It's meant, explicitly, to combat against racial discrimination.
That's the nature behind the Rooney Rule. It asks the hiring managers/interviewers to reflect upon their interviewee's and recognize that they may have an inherent bias against hiring a marginalized group, reflect upon their selections, and do their best not to hire based on race (and in this implementation: sex).
In practice it's more like adding a quota or you may be accused of not following the rule. But I digress and don't want to argue into too far into that territory, as it becomes speculation of human motives.
To quickly sum up my thoughts on why it is essentially a quota in practice: If you do not hire more marginalized groups, you'll be accused of not honoring the Rooney Rule.
In order to comply with the unofficial Rooney Rule and make your manager happy, "You're increasing diversity! Good job!", you simply make sure to hire 1-2 marginalized people out of every 10 you hire.
But of course the "in good faith" argument applies and you can go back and forward bickering over whether they were hired in as a quota or "in good faith".