The utility in POSIWID is that you don't have to get lost in the discussion about what a system is supposed to do or was originally designed to do. If you're telling someone "the US was intended to be a property-owning oligarchy from the beginning" then you are correct, but they can argue with you about intention and changes over time and blah blah blah. If you start from describing it a as it currently exists, then you don't need to talk about intention or political parties or the current SCOTUS judges or whatever. You can point out that it is a property-owning oligarchy, and the various processes that make up the system are producing that outcome.

It's a way to short-circuit normative arguments and jump to the descriptive. It's important sometimes to point out the history and context of a given system so that it's clear why there's a disconnect between what a system does vs what people think it's supposed to do, but other times you just want to get to the point that a system isn't doing what people think it should do because it's not designed to do it, no matter what changes you make to the variables or who you designate to run the system. A lot of people like to talk about broken systems vs bad systems vs "this system just needs the right people running it" and POSIWID is a way to cut through all that and talk about the fact that the system is the system. If it's doing a thing, it's because that's what it does.