Here's why:
* Nonrestrictive means the information provided by the clause or phrase is extra, not essential to understanding the main idea of the sentence. It could be removed without changing the sentence's core meaning.
* Commas signal a pause or a break in the flow of the sentence, indicating that the information is separate from the main clause.
Examples:
* My dog, who loves to play fetch, is very energetic. (The phrase "who loves to play fetch" is nonrestrictive because it's additional information about the dog.)
* The house on the hill, with its white picket fence, is beautiful. (The phrase "with its white picket fence" is nonrestrictive because it describes the house in more detail but doesn't define it.)
Remember: If the clause or phrase is restrictive (meaning it's essential to the meaning of the sentence), you wouldn't use commas.