We have a tendency to want to know exactly. What is the exact value of this action? how much effort is too much for this end etc.
Even in feelings. How much do I want to care on X? etc.
Usually we have quite an idea about the amount more or less, but then some tend to want to know exactly.
Yet knowuing exactly is usually impossible. Because of hte complexity of things and so on.
Moreover, the utility of finding out the exact is deteriorating, while the cost of finding the exact go up.
PS. there is another cost of closing the mind with having a clear cut number. Also, see Jon Elster about people preferring to rely on rationality.