It just adds an extra step that keeps the general public away from your PII. The post office knows who owns that PO box and where that person actually lives. A simple subpoena makes that information available to whatever arm of enforcement needs it.

I've been using PMBs for about 20 years. If you don't own real estate it can definitely help anonymize your location, even to the government. You rent one home, get a PMB using that location as your home, then move. Use the PMB for everything and never give out your new home address.

If you own real estate it's much harder, especially now that companies have to report their owners and directors.

At the very least PMBs help protect your home address from companies and the public.

PO boxes have stricter address information requirements but there are other services that are more lax and can pass with a utility bill from a previous rental address. Thus, obfuscating the real address.

A lot of rich and famous do this, keeps the “riffraff” off their ass. Not meant to be bulletproof but can be useful.