In this sentence, the comma is used after a long introductory phrase. Essential clauses do not require commas. In this sentence, the clause ‘when the police superintendent’s messenger overtook him’ is an essential clause because it provides essential information. Hence, a comma is not required in this sentence.
In this sentence again we have an introductory clause which provides extra information. The second half of the sentence can stand alone and, therefore, is separated from the introductory clause with a comma.
What is the Planning Process?