The answers tends to be either finding a way to eat the inedible (because there are times where calories > no calories regardless of the source of calories), or finding a way to conserve it somewhat / somehow. Even ignoring its use in creating alcohol from sugars, fermentation can do both (separately or at the same time) and more.

Often these become typical delicacies (the flavors can be intriguing or interesting, or "interesting" in the case of some like Hákarl and possibly the Hongeo-hoe you're talking about).

For Casu marzu, it's probably either finding a way to control spoliation such that the result is still edible, though it's also possible that uncontrolled spoliation simply resulted in something those adventurous (or desperate) enough to try eating it took a liking to.

I'd expect the latter to be the story here: hard cheeses like Pecorino keep a while until they're "open" and you start eating them, and it's not a huge cheese (I believe pecorino wheels are under 2kg, we're not talking the 30+ wheels of a parmesan or comte).

Or maybe sardinia regularly has conditions where the cheese can't be kept and so a controlled decomposition was superior to just losing the cheese.