Experience Rather Than Knowledge
"Knowing" and "doing" are two very different things.
For example, it is quite difficult to predict the rate of shrinkage during the sintering of a ceramic substrate. Even if you fire ceramic pieces under the same conditions as described in chemistry manuals, the actual results often differ from theory. Unless knowledge is backed by actual experience, you cannot obtain the right result.
This applies to management as well, be it sales or administration. Only experience can make knowledge and theory viable.