Lots of systems are based on "defence in depth", i.e. multiple layers of security. This approach acknowledges that any single layer of security will have holes

Lots of systems are based on "defence in depth", i.e. multiple layers of security. This approach acknowledges that any single layer of security will have holes somewhere just waiting to be found no matter how theoretically secure you think it is. The implementation matters, not the theory and quite often this is where the holes exist. "Obscurity" is just one of those layers. Given no security, e.g. wide open front door, adding a door will reduce the number of successful attackers. Having a lock & key is a level up, hiding the key would reduce the chances of a successful burglary even more. Armed guards on the door would again reduce the chances of you getting burgled. Adding barbed-wire around your house improves the situation again... and it goes on. None of those levels of defence are impenetrable, but they all help. "Security by obscurity alone is discouraged and not recommended by standards bodies" (https://en.wikipedia.org/wiki/Security_through_obscurity https://en.wikipedia.org/wiki/Security_through_obscurity)