Bugs get fixed when systems are iterated on. They also tend to be single results from single mistakes, not compound end results of the implementation.

Design features tend to persist.

The phrase/idiom "the purpose of a system is what it does" maps best to situations where a multiple decisions within a system make little sense when viewed through the lens of the stated purpose, but make perfect sense if the actual outcome is the desired one.

It is an invitation to analyze a system while suspending the assumption of good faith on the part of the implementors.