Parenthesis is inserted into a sentence when one is qualifying or elaborating on a certain idea or topic. It can be set off by commas, dashes, little lines, and brackets. This literary device serves a number of purposes such as providing extra, detailed information or giving an afterthought to a strong point made before. It can also interrupt the flow of a sentence which serves the purpose of employing a natural, spoken, and informal feel to a sentence.
“‘Black dog’ is the mood of bottomless, suicidal despair suffered, most notoriously, by Winston Churchill (himself a kind of bulldog in nappies, a logo for Empire; growling and dribbling, wheezing smoke, swollen veins fired with brandy).”
(“Lights Out for the Territory” by Iain Sinclair)