The HR team is the company's first line of defense from disgruntled employees. If you submit a complaint to HR, it is directed to the Employee Relations team. Someone from that team should schedule a call with you to discuss your complaint. They will then run the details by the firm's employment lawyers, so the lawyers can evaluate it for ethics/code of conduct violations and legal liability. More often than not, they will decide that your complaint has no teeth, and the ER team will provide you with a canned response to that effect. It is then up to you to pursue other avenues, e.g., get your own legal representation. I don't know of any examples of HR actually helping an employee as a result of a complaint. If anyone does, do tell.
Bumped from @4fuy+1djb89BH for being on point.