Parenthetical and transitional expressions often interrupt the flow of a sentence or digress, so they are usually set off with commas. Parenthetical expressions (in fact, by the way) add comments. Transitional expressions, including conjunctive adverbs such as however and furthermore, clarify how parts of sentences relate to what has come before them.

Roald Dahl’s stories, it turns out, were often inspired by his own childhood.

Ceiling fans are, moreover, less expensive than air conditioners.