For centuries thinkers have assumed that the uniquely human capacity for reasoning has existed to let people reach beyond mere perception and reflex in the search for truth. Rationality allowed a solitary thinker to blaze a path to philosophical, moral and scientific enlightenment.
Now some researchers are suggesting that reason evolved for a completely different purpose: to win arguments. Rationality, by this yardstick (and irrationality too, but we’ll get to that) is nothing more or less than a servant of the hard-wired compulsion to triumph in the debating arena. According to this view, bias, lack of logic and other supposed flaws that pollute the stream of reason are instead social adaptations that enable one group to persuade (and defeat) another. Certitude works, however sharply it may depart from the truth.
The idea, labeled the argumentative theory of reasoning, is the brainchild of French cognitive social scientists, and it has stirred excited discussion (and appalled dissent) among philosophers, political scientists, educators and psychologists, some of whom say it offers profound insight into the way people think and behave. The Journal of Behavioral and Brain Sciences devoted its April issue to debates over the theory, with participants challenging everything from the definition of reason to the origins of verbal communication.
New York Times: Reason Seen More as Weapon Than Path to Truth
This evolutionary psychology explanation is (like most evol pysch) speculative. Regardless of whether reason evolved “for” the purposes of argument, or merely reached a point where it was flawed but “good enough” we may never know. But I do think most people use reason more to defend their positions rather than to arrive at accurate positions (what does that mean for me, and my arguments?) To quote Michael Shermer in Why People Believe Weird Things, “Smart people believe weird things because they are skilled at defending beliefs they arrived at for non-smart reasons” (via that Cracked article).
See also Systematic Ideology and Cultural Cognition.