[dredmorbius]

This piece explicates the concept well IMO:

[Beer] frequently used the phrase “The purpose of a system is what it does” (POSIWID) to explain that the observed purpose of a system is often at odds with the intentions of those who design, operate, and promote it. For example, applying POSIWID, one might ask if the purpose of an education system is to help children grow into well-rounded individuals, or is it to train them to pass tests? “There is after all,” Beer observed, “no point in claiming that the purpose of a system is to do what it constantly fails to do.”

<https://www.forbes.com/sites/benjaminkomlos/2021/09/13/the-p...>

(Much more in the essay, but that's a gist of it.)