When we say "I trust this expert's opinion on this topic because he knows more than I do." we are doing something amazing, or at least it's amazing when that opinion goes against our own. We are exercising a skepticism about our own reliability.
Our beliefs about the world tend not to change. We think our current view is about as correct as anything out there. Since reality is filled with an infinite set of unknowns, the areas of our own ignorance about which we are actually aware, is quite a small sliver. Combined with the fact that our brains can only inhabit one interpretation at a time, its vital that we maintain this uncertainty. Let's look at a simple, concrete example of this limitation.