Consider some of illusion at the bottom of the demo page (click here to see it).
The two middle color patches look as if they are different, but in fact they are exactly the same. What is gong on here? How can it be that we see wrong? How can it be that eve after we are shown that these two patches are identical we still can’t see them accurately?
It is because our brain is wired in a particular ways and this wiring, while very good for some things, is not perfect — and it makes us susceptible to certain errors and mistakes. Moreover, because these mistakes are a part of us, we are fooled by them in predictable and consistent ways over and over.
Now, vision is our best system. We have lots of practice with it (we see many hours in the day and for many years) and more of our brain is dedicated to vision than to any other activities. So consider this — if we make mistakes in vision, what is the chance that we would not make mistakes in other domains? Particularly in domains which are more complex (dealing with insurance, money, etc.), and ones in which we have less practice? Domains such as decision making and economic reasoning?
Not very high I think — and this is why we have lots of decision illusions. The predictable, repeated mistakes we all make in our financial, medical, and other daily decisions.