Understanding is important. Knowledge is important. Knowing sure knowledge is important.
When we take action we have to know for sure so as not to make any miscalculations. So what is the knowledge that we must be after? Sure knowledge. This is called yaqin. I cannot explain how important it is that you must be after yaqin, sure knowledge, certainty. You have to know FOR SURE.
When you do your action with knowledge, then you can do it with more conviction because you know it is for sure. When you do an action, you may wonder if it is the right action or if it is wrong. You may feel a little scared. To do the action you must know that it is the right action with the sure knowledge you have. Still you should leave 10% to the fact that there are things you do not know.