Because you can't know what's going through the players' heads, PbtA games give you mechanics to drive the story based on what the players show an interest in (as reflected through their moves, questions, etc) rather than what the GM thinks might be cool. The GM still populates the world with believable monsters and NPCs, but the players have much more control over how they engage with the fiction in a PbtA game than they might in a more traditional RPG. If you've run a lot of very "sandboxy" games/settings in the past, this may not feel like as much of a change. But if you've never played a game that gives the players as much low-level agency over the direction of the story, it's a huge shift.

This perfectly encapsulates why I've been loving PbtA games so much. By not prepping very much, you keep the story tightly bound to the interests of the players, and also can have very little idea what is going on as GM. In D&D (as the DMs I've played under do it, at least), when somebody does something, they consult their prep and, half the time answer with "Well, you don't accomplish anything". They knew everything that was going on, and whatever the player did failed to be related. In anything PbtA, a player investigating X should cause X to be important in some way. Which creates a vastly more pacy experience in which every player is guaranteed to feel as involved regardless of the quality of their ideas or rolls. It also allows you to truly "play to find out what happens" and have a story spill forth from the dregs you actually planned, which is incredibly exciting.

The main purpose of the rules is allowing you to keep things moving in a fun direction, where you'd have no clue how to do so otherwise. I think this might tie into some of the problems I'm seeing here? Because that is the main purpose of the GM rules in every PbtA game -- to provide direction without prep -- not to restrain or limit.

A pyrrhic victory is one in which the cost was so high that the victory itself is pointless. In other words, only a victory by technicality (e.g. You captured the generator...but your gang died in the process, so you don't need the electricity anymore). Which would only be fair in the case of a miss: A 7-9 is fundamentally a victory, while a pyrrhic victory is fundamentally failure.