I see it like debug statements in code. If the only access you have to a function is from the inputs and returns, it's difficult to see where the issue lies when the return value is wrong. Having an internally accessible layer of I/O lets you bypass certain areas or get values at different steps to determine root cause. Could be useful for firmware testing, fixing ones that fail a normal test, and RMA's.

You would want to leave the circuitry in place for production because it's largely unnoticeable and changing it could introduce bugs.