Commas are used for many purposes. That’s one reason why they are so confusing to so many people. However, their primary purpose is to improve reader understanding, and the primary way they do that is to separate elements of sentences into meaningful parts.
Sometimes, commas identify parts so you can see how they are connected. Sometimes they identify parts that are not essential to understanding the main message of the sentence, which means the reader can figure out what you mean to communicate.
Let’s look at that second use a bit more and see how it works.
Here’s our main message we wish to communicate: The governor passed the bill to ban drunk driving.
Now let’s add some description: After much deliberation, the governor passed the bill to ban drunk driving.
The comma lets the reader know when the non-essential description is done and when the main point is about to start.
Now let’s add some more: After much deliberation, the governor, now in his second term, passed the bill to ban drunk driving.
The phrase now in his second term is not essential to understanding the main point of the sentence. It’s not even on the same topic! The commas around that phrase tell us (1) when we divert from the main idea and (2) when the non-essential information is complete and we’re about to return to the main idea.
The overall effect of those three commas is the reader can easily identify the main idea of the sentence. With the commas in the correct places, we are able to communicate our ideas.
That’s why the commas have to be right.