They love it because they were instructed to use it in school, and they love it because it supposedly reduces ambiguity, as demonstrated by several popular examples.
They use it religiously, except for journalists, who are forced by AP Style to omit it - a specification I always attributed to journalists caring more about concision than clarity. (Business Insider is a rare publications that insists on the serial comma, since our editor in chief learned to love it in his days at Yale and on Wall Street.)
In case you slept through English class, I'm talking about the comma after the penultimate item in a list of three or more items and before the conjunction, as in "dogs, cats, and birds" as opposed to "dogs, cats and birds."
I once loved the serial comma, too, until a recent realization caused me to look closer and discover that the punctuation mark is massively overrated.
The topic came up while I was talking about Christina Sterbenz's article on commas with a friend who works at a top-five New York law firm. Surely, if anyone cared about precise language it would be a corporate lawyer; but I was shocked to hear that he opposed the serial comma and incredulous when he claimed that his firm's style guide omitted the serial comma. I tried to convince him that he was mistaken, but after going through every argument I could think of or find online, I had only started to question myself.
Later my friend showed me that his firm's style guide did indeed omit the serial comma, as it concluded that the punctuation mark was a stylistic conceit of publishing houses, unnecessary for precise language, as demonstrated by places like The New York Times, The Wall Street Journal, and The Economist. On instances where a serial comma could be added to reduce ambiguity, the style guide allowed that it could be added, as long as usage was consistent across a document.
In case you think that one firm may have gone insane, another corporate lawyer I know said that his firm also did not insist on the serial comma.
And so I turned to the arguments in favor of the serial comma and saw how easily they crumble.
The preeminent argument in favor of the serial comma involves a very limited form of ambiguity. It has been popularly illustrated and demonstrated with the following sentences:
"We invited the strippers, JFK, and Stalin."
"We invited the strippers, JFK and Stalin."
Grammar nuts will rightly point out that the first sentence is clear, while the second sentence could be thought to mean that JFK and Stalin were the names of the strippers. In grammatical terms, the second sentence leaves ambiguity as to whether "JFK and Stalin" are names on a list or an apposition describing "strippers."
The problem with this argument is that the serial comma could just as easily, if not more easily, create ambiguity in a similar example:
"We invited the stripper, JFK, and Stalin."
"We invited the stripper, JFK and Stalin."
In this case, the first sentence could be thought to mean that invitations were sent to Stalin and a stripper named JFK, while the second sentence is technically clear. Simply changing the word that precedes the ambiguous pairing from plural to singular puts the serial comma on the wrong side of clarity. And the same holds true for any of the popular examples that have circulated over the years.
Examples of problematic serial commas are easy to come by, such as: "I went for a walk with Betty, a nurse, and a cook." In that sentence it is not clear whether you went for a walk with three people or Nurse Betty and a cook.
In short, there is a small risk of ambiguity with or without the serial comma. With either it is possible to resolve ambiguity with simple rephrasing:
"We invited JFK, Stalin[,] and the strippers."
Another example cited in favor of serial commas is the following scenario:
A will bequeaths a certain amount of money to "John, Bill and Steve." John insists that the lack of serial comma means half of the money should go to him and half should be split between Bill and Steve.
When I brought up this example to my lawyer friend, he wasn't having it, saying that he would never interpret the case as such. For "John's" interpretation to be at all conceivable, one must assume that the person who wrote the contract had very awkwardly failed to include a conjunction between two separate parties of "John" and "Bill and Steve." Anyway, this rare ambiguity would be resolved by any rational person through simple rephrasing: "half to John and half to Bill and Steve."
And that's it. Those obscure cases are the only examples I found where the lack of a serial comma creates ambiguity, and just as many obscure examples can be found to show where its presence creates ambiguity. In any case, ambiguity can be easily resolved by rephrasing.
That's why many law firms and most newspapers don't care about the serial comma.
The grammar snob's favorite mark is just a waste of space.
(Nonetheless, students, publishers, and employees of Business Insider must abide by their style guides.)