I can think of some (admittedly contrived) scenarios where being allowed to discriminate on illegal-in-the-US traits would be beneficial to the company. For instance, I think women would be much more willing to buy undergarments from women than (especially, unattractive) men. There are definitely people with a preference about the sex of their doctor. There are lots of documented studies about attractive people being more successful in management and sales, so especially if the hire is viewed as a long-term career move, hiring an unattractive person might be objectively inferior.

There are specific, legal, questions you could ask which would be ok ("how many years of experience do you have with bras?", various technical questions, etc.) which might correlate highly with certain physical characteristics.

I think there is a moral argument against discrimination on these grounds, but there's also an argument for letting companies make their own decisions.

Even without legal prohibitions on discrimination, I wouldn't want to create a workplace where this kind of discrimination happened -- even if it were good for a specific job, it would also drive away other candidates for other jobs, and I think the net loss would be greater.