Lloyd Jones quoting revivalist Jonathan Edwards to this effect, who had said “the main benefit obtained by preaching is the impression made upon the mind at the time, and not by an effect that arises afterwards by remembrance of what was delivered.” Edwards maintained that the greatest benefit of preaching was the impact it had on the person at the time when they heard it, as opposed to looking back over notes in order to recall what had been said. People will most often be able to recall specific events if they have strong emotions attached to them. Likewise, when people think back to how they felt when someone preached a powerful word, then that message has another opportunity to work in them for transformation.
Philip Greenslade (Ed. Greg Haslam)
From the book "Preach the Word"