When you are driving and you notice a traffic sign, what do you do? If you are like most drivers, you divert your attention away from the road, long enough to interpret the sign. In other words, you slow down.
When you are reading and your eye comes across punctuation, what do you do? If you are like most readers, you divert your attention away from the text (the words), long enough to interpret the punctuation. In other words, you slow down.
What are the implications of this, with regard to writing?
If you were building a road and your purpose was to achieve a smooth flow of traffic along that road (and you had no safety concerns to deal with), how many traffic signs would you put up? Most likely, as few as possible.
If you were writing an essay and your purpose was to allow the readerís eye to flow as smoothly through the essay, from beginning to end, as possible, how much punctuation would you use? The answer, of course, is: no more than absolutely necessary.