All punctuation exists, basically, to help readers understand what you wish to say. Mostly marks of punctuation do this by signaling the grammatical or logical structure of a sentence (usually these are the same):
In the long history of the world men have given many reasons for killing each other in war: envy of another people's good bottom land or of their herds, ambition of chiefs and kings, different religious beliefs, high spirits, revenge. Rulh Benedict
The colon divides this sentence into its two principal parts: the introductory generalization and the list of specific reasons. The commas within the list mark each single reason. The period closes the total statement.
Less often punctuation marks stress an important word or phrase:
In 1291, with the capture of the last great stronghold, Acre, the Moslems had regained all their possessions, and the great crusades ended, in failure. Morris Bishop
Bishop does not need the comma before the closing phrase to clarify the grammar or logic of the sentence. Its purpose is isolate and thus stress the phrase. (The other commas in the sentence, however, function in the more usual way, indicating grammatical and logical structure.)
Finally, punctuation may mark rhythm. Listen to this sentence closing an essay on General Robert E. Lee:
For he gave himself to his army, and to his country, and to his God.
The commas separating the coordinated phrases have no grammatical necessity. In such coordinated series, commas are not usually employed with and. Here, however, the requirements of a closing sentence—that it be slow and regular in its the commas.
Of course, these three functions of punctuation often overlap. Sometimes a comma or dash both signals grammatical structure and establishes emphasis. And anytime you put a comma into a sentence to help readers follow its grammar, you automatically affect emphasis and rhythm.
Still, keep in mind that these different reasons for punctuation exist. Asking yourself an unspecified question like "Is a comma needed here?" is not very helpful. Rather you must ask: "Is a comma needed here to clarify the grammar (or to establish a particular rhythm or stress)?" About Bishop's sentence we can answer that the comma before "in failure" is not required by grammar but is necessary for emphasis.
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