Why are nonrestrictive clauses used?
Nonrestrictive clauses are used to give extra, nonessential information about a noun, pronoun, or noun phrase (e.g., “The summer Olympics, which take place every 4 years, will be in Paris”).
Unlike restrictive clauses, which are used to identify people or things, nonrestrictive clauses are typically used to give additional information about particular people or things. So, they are frequently used with specific nouns (e.g., “my mom,” “the new teacher,” “your house”) rather than general nouns (e.g., “kids,” “the man,” “books”).
When a relative clause is used to describe a proper noun (e.g., “David,” “Prospect Park”), a nonrestrictive clause is almost always used.