If people are so typically bad at reasoning effectively, why study logic, formal or informal? Why have normative standards at all? What's the point of having ideals for argumentation if they are so seldom met in practice?
Because those standards point toward ways to improve our practice, moving us closer to that ideal even without actually reaching it.
Argumentation doesn't have to be perfect to be effective, so to think we shouldn't bother at all because the process isn't infallible, so it's worthless, is to commit the Nirvana fallacy.
Of course we often fall short, but there's nothing at all wrong to have standards and ideals to point the way, making us better negotiators and more skilled at reaching reliable conclusions.
And that's even without a mistaken and pointless need for absolute certainty in worldly matters of fact.
Of course we make errors in our thinking, and in reasoning among ourselves. Of course we are biased and often invested in a given conclusion.
That's the point.
The point of it all is to recognize our biases, our motives, and the fallacies in our own arguments that result from those, and not despair of never reaching complete closure on our knowledge through impossible standards of metaphysical certitude.
Because it's the process itself, not the conclusion that we may have a vested interest in defending, that in my view matters in seeking out the facts of any matter, and what truth those facts bear out (to steal a page from Bertrand Russell).
Because the greatest enemy to seeking and sometimes finding that truth lives inside each of our skulls, not just those of our ideological opponents.
And you cannot overcome a foe that you do not recognize or notice.