It is/was a useful credential to have when responding to government RFPs as governments had purchased UNIX systems and wanted new systems to be interoperable. This might no longer be an advantage in most requests but my guess is there are still some that require it.
I don't really know what the certification gets you these days other than bragging rights. I imagine in the past, government and corporate customers had check lists that included a box for "certified Posix", but I'm not sure if that is still the case.