Philadelphia , Pennsylvania , is a neat town. Jim Jones , Professor of English , entered. Similarly, commas are used to insert the name of a speaker within a quote, a special form of insert. Inserts also offer a way to stress elements within a sentence. Speakers can also introduce a pregnant pause that draws attention to last word uttered and introduces anticipation of the next idea.
Writers can achieve somewhat the same effect by introducing inserts.
My brother is a chemist. My brother, you recall, is a chemist. The comma before an insert introduces a visual pause that stresses the preceding word. Even a blank insert would introduce the same effect. An additional form of insertion, when we feel we have to insert something but do not want to break the flow at all, is the footnote.