I was looking up vacuum tube recently. Apparently there are things called "getter" pins in them that connects to internal material deposition device, used to remove last bits of air out of tube in the manufacturing line. The exact behaviors and operating parameters for those pins are corporate secrets and not documented. Datasheets only mention them as "No Connect" pins that do nothing and has to be disconnected from anything for safe and correct operation of the device. BACKDOOR!!! in an analog vacuum tube.

... everything ever always had factory-use-only interfaces. Manufacturers don't document every bits of everything on public datasheets. Granted, the dynamics of this change if they can be used as backdoors, and those might have to be either disclosed or securely disabled, but "why they exist in the first place" is just nonsensical question; they exist for necessary internal purposes.

That question should be "how should factory use interfaces be handled going forward".