Modifiers, whether adjectives, adjectival clauses, adverbs, etc, are functioning to restrict the range of reference of the words they are modifying (be it nouns or verbs). Correct? That's the whole point of modifiers: they are restricting the meanings of the words they're attached to.
So can somebody explain how some modifiers can be non-restrictive, and thus need to be set aside by commas? Surely if a modifier is not restricting the meaning of the word(s) it's attached to—the very definition of a modifier—it's not a modifier. Am I just visualizing this all wrong?
I don't understand the reasoning that the non-restrictive is just not essential for the rest of the sentence to make sense, which is the usual answer I receive when I search for an answer to this Q online.
If anybody could point me in the right direction, I'd much appreciate it.