PPC Wiki

Critical Reasoning is an important ability for any Agent, be they a Bad Slasher, Assassin, Floater, or Pyro. Critical reasoning is, to steal from Wikipedia, the purposeful and reflective judgement about what to believe or what to do in response to observations, experience, verbal or written expressions, or arguments.

In other words, critical reasoning is the most important tool any Agent, nay, defender and supporter of the Canon Protection Initiative can have, for without it, all is for naught, and friend cannot be distinguished from foe, and the difference between goodfic and badfic loses its meaning.

Critical reasoning is also important for Boarders as well; it is the best way to keep yourself from looking ridiculous unintentionally.

In many canons, critical reasoning is an important part of the characters and is required on the reader's part to understand the plot or story, or even the characters' viewpoints themselves. In others, you should just turn your brain off and enjoy the ride if you can. This isn't to say that it's necessarily bad for a story or series, it just means that there are aspects of the story (in some measurable quantity) where trying to understand the logic or reasoning used in the canon will just cause your brain to overheat and explode trying to compensate.

(That said, you should really read the entire article over there, it is quite interesting.)