> what exactly is the business case?

Ethics is also a containment vessel internally.

With an Ethics dept, concerned employees can rest easy knowing someone else is responsible, or if they raise concerns, they have someone to route and advocate their concerns, rather than adding noise to the chain of command. If the dept. ends up ineffective after some years, employees can blame the department, or have renewed hope when personnel turn over. So (slow) turnover is good from this perspective.

there's probably a box to check on the "be attractive to investors" form and then you can also put the ethics department in HR docs as an escalation path to make emo employees feel better. I bet the ethics people get told how important they are when hired and the team is stood up but then get upset when they're continually bypassed and quit. I bet the bait/switch hurts a lot.