Ignorance
There's at least a couple of kinds of ignorance: the ignorance that we recognize and the ignorance that we don't recognize. Ignorance that we don't recognize is generally harmful and another cause of suffering; after all, when we believe that we know something, we can no longer learn more about it. Conversely, ignorance that we recognize is potentially beneficial.
First, let's specify the kind of ignorance we're talking about. The ignorance we're interested in here is not something like the ignorance we all have regarding most subjects. Most of us are completely ignorant of nuclear physics, for instance. Here instead, we're interested in our ignorance of our daily life. How do we behave, why do we believe what we do, that sort of thing.
Why is this ignorance important? Simply, if we are ignorant of our behavior and the foundation of our beliefs, we miss our chance to eliminate most of the causes of our suffering. If we are unhappy but persist in the same behavior, we are likely to remain unhappy. It is only through watching ourselves and finding the ultimate causes of our unhappiness that we can have a chance to reduce our suffering.