Generally, punctuation can help the reader discern the difference between the control unit and the support unit/s. Commas etc, are the marks which show what belongs in what unit in a sentence.
- Let’s eat Harry!
Here, there is no punctuation, and so the reader sees that the object of the verb ‘eat’ is Harry. We can ask the question: whom shall we eat? The answer is Harry. If there is a comma after “eat” then the reader knows that the expression. “let’s eat” is one unit of thought, and is separated from Harry, which is seen as an aside- he is being addressed and asked whether he wants to eat.
- When feeding a shark often mistakes undesirable food items for something it really desires.
Without punctuation, it is unclear whether the sentence means, “when [you] are feeding a shark….”
- When I last heard of him he was still acting naturally.
- When I last heard of him he was still acting, naturally.
Here, without punctuation, the first sentence means that he was acting naturally (like a normal, unpretentious person). The second one says that she was acting (i.e. posing, pretending) and the “naturally” becomes a sarcastic aside which implies: “Well, that is what I would expect from such a pretentious person”