So comma rules fall into these two categories: Every time you use a comma, you are either separating two elements that should not bump up against each other, or you are using a pair of commas to surround an element.
It is in this second category that we need to understand some terminology. We get into the habit of thinking that, when we surround something with commas, it is "nonessential." This is just not the best way to think of these elements. We need to be careful about this term nonessential. It does not accurately describe an overall reason for using a pair of commas around a sentence element.
...We arrived in Springfield, Ohio, late in the day....
Is Ohio "nonessential"? Isn't there a Springfield, Illinois? And yet we definitely surround the state when it follows the city -- and not because it is "nonessential."
...It happened on May 19, 2012, early in the morning....
Is 2012 "nonessential"? Aren't there other years with a May 19? And yet we surround the year with a pair of commas when it follows the date -- and not because it is "nonessential."
...I spoke, Mr. Parker, with her sister....
When we address someone, there is the chance that we are using the name to distinguish which person we are speaking to. Doesn't that mean we need his name in the sentence? And yet we surround a name used in direct address with a pair of commas -- and not because it is "nonessential."
So to say that an element that is surrounded with a pair of commas is automatically "nonessential" or "not necessary to the meaning of the sentence" is just not accurate. We need a much better grasp of why we surround elements with a pair of commas.
More to come....
Happy punctuating!
Margie