An old friend of mine expressed confusion and dismay recently, that the rationalist movement is as concerned with cognitive biases as it is. Given that being aware of biases does not significantly reduce susceptibility to them, he believes identifying useful heuristics might be a better approach.
I have a heuristic that I like: "If something seems totally unfit for its purpose, you have probably misidentified that purpose."
Two examples: The function of a typical "dysfunctional" civil service bureaucracy is not to complete any nominal mission on behalf of the public, but to provide stable, low-risk jobs and petty administrative fiefdoms for those inside it; to fulfill the material and psychological needs of the people making up the organization. It performs this task admirably. The function of the public school system is not to provide learning of any useful material; its primary function is instead to grind down and separate those who are docile and trainable from those who are not, for use as raw material by industry and the state.
I work in computer security. I like doing offense; you only have to be right once, while defense has to be right every time. My impression of Thinking Fast and Slow was "This book is a weapon. No, an armory."
The function of studying biases is not to make our reasoning clearer. The function of studying biases is to win arguments, make sales, and otherwise influence the behavior of others. It serves the social exercise of the will to power, not the pursuit of truth (this is true of philosophy generally. Socrates was a glorified mugger armed with the cudgel of reason). In the weakest case, the study of biases provides a feeling of superiority over others whom you witness falling victim to biases. I am a cynic about human nature, and the sort of people who end up at LessWrong are, by my impression, not especially well-adjusted, high-status, or spectacularly successful people (nor am I). Understanding cognitive baises, identifying them in others, and feeling superior serves a psychological function. I really don't think there's much more to it than that.