My brother who is in the first grade had a headache. State identifications and titles function much like inserts, with a comma before and after. 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.