You can even reasonably guess someones salary based on their job and location, especially a coworker's.
Precisely the people with hard-to-guess salaries may value that privacy.
- an insurance company demanding a premium because you're well paid
- competitor bidding for your labour anchoring their offer to your current salary
Those are just two cases were it could be harmful. The idea is to make the information available and transparent while still not making it public and still protecting it.
I'm not a bad actor, who spends all their time figuring out how to make profit off of someone else's private info. I'm pretty sure there're very creative ways private information (salary and not) can be misused, but I don't want to find out by volunteering this information.