Because there’s a wrong way to do things and we need to defend it.
“Our hypothesis is that the function of reasoning is argumentative. It is to devise and evaluate arguments intended to persuade.” [DS: good]
“Skilled arguers, however, are not after the truth but after arguments supporting their views.” [ehh, sorta]
“Reasoning so motivated can distort evaluations and attitudes and allow erroneous beliefs to persist. [that’s poor reasoning and should be discounted from pure theory]
Proactively used reasoning also favors decisions that are easy to justify but not necessarily better. In all these instances traditionally described as failures or flaws, reasoning does exactly what can be expected of an argumentative device: Look for arguments that support a given conclusion … favour conclusions for which arguments can be found.”