There is significant business incentive to work with an attorney when navigating complex employment situations. Should litigation arise, your discussions with your attorney are privileged, meaning that parties generally cannot request that you disclose what you've talked about. This protection does not exist with HR consulting firms.
The Attorney-Client Privilege is one of the "sacred" bedrocks within the legal system. This longstanding principle provides one of the highest levels of confidentiality in our society. Not even a court of law can require disclosure of information covered by this right.
More information about the Attorney-Client Privilege and confidentiality can be found in the California State Bar's Rules of Professional Conduct.