The purpose of a system is what it does (POSIWID) is a systems thinking heuristic coined by Stafford Beer,[1] who observed that there is "no point in claiming that the purpose of a system is to do what it constantly fails to do."[2] The term is widely used by systems theorists, and is generally invoked to counter the notion that the purpose of a system can be read from the intentions of those who design, operate, or promote it. When a system's side effects or unintended consequences reveal that its behavior is poorly understood, then the POSIWID perspective can balance political understandings of system behavior with a more straightforwardly descriptive view.

The point of POSIWID is to properly evaluate utility, by pointing out that the intended purpose for something is irrelevant to its actual effect. We’re not disagreeing, but utility simply is not synonymous with purpose, because the phrase is meant to counter the assertion that the intended purpose of a system is relevant to its effects in the real world. If you replace it with “utility” then you’re basically saying “the utility of a system is what it does”, which is true, but also redundant and not the point of the phrase.