The non-disclosure also benefits the person making the complaint. I can assure you that many companies will hesitate if not flat refuse to hire a person making a sexual harassment complaint in a previous job simply because the potential employer has no way of knowing whether the complaint is legitimate or if the woman is a troublemaker. Why hire a potential problem?
I can also tell you both as a legal assistant that spent 17 years in the field and as an adult woman who went back to college for a degree and moved into a male-dominated field that 99.9% per all "sexual harassment" complaints are made (sadly) by women gaming the system. Working in two male dominated fields I dealt with a number of "offers" without ever having to resort to either HR or physical violence. I also learned how to read a man's actions and respond accordingly to keep our relationship at the proper temperature.
Truthfully, I think it's pretty sad that young ladies are not educated to understand the power of their femininity. It seems to me that they are taught to regard the males in a hostile manner. For the most part, if you appeal to a man's finer qualities, he will respond in a positive way (not always, but mostly).
I agree that secrecy is an advantage to the complainant as well, pretty much the advantage you describe. Generally speaking, sexual harassment complaints are confidential. But the secrecy agreement at issue here is a contract, payment is conditioned on the complainant dropping recourse to the courts, and on complainant maintaining some (but I doubt total) secrecy.
A settlement NDA doesn't make the complaint secret, and it can only bind the parties to the agreement. If the case is filed in a public court, the fact that there was a compliant is not secret.
From a past life, I understand that asking an interviewee questions about age and family status is off limits - I wonder if the same is true about asking about harassment claims.