In English, writers and speakers use expressions not really needed to convey the basic thought of particular statements, but these expressions give a desired texture, help establish mood and context, or capture the spirit of spontaneity that marks most day-to-day human interactions. Known as parentheticals, they are of two kinds: (1) expressions that are used again and again as side remarks, and (2) non-essential words and phrases — even complete sentences — that interrupt the flow of a sentence to explain or amplify a point.
Most of us are familiar with these often used parenthetical expressions: "as a matter of fact," "of course," "indeed," "moreover," "therefore," "by the way," "on the other hand," "nevertheless," and "in my opinion." They emphasize or direct the reader's or listener's attention to what's about to be said or being said, or serve as transitional devices to make statements clearer and unmistakable.