Commas: those little morsels of goodness that few authors understand. In general, serial commas are used to resolve ambiguity. When we have a list in a sentence, not using commas can create some interesting situations.
Comma use is part of what we call ‘style:’
Google says: “Style is the way writing is dressed up (or down) to fit the specific context, purpose, or audience. Word choice, sentence fluency, and the writer’s voice — all contribute to the style of a piece of writing.”
To my parents, Ayn Rand and God.
There is ambiguity about the writer’s parentage, because Ayn Rand and God can be read as meaning that the writer claims Ayn Rand and God are the parents. That is actually rather hilarious because Ayn Rand is famously atheist in her beliefs. (I’m not qualified to say whether or not God believes in Ayn Rand.)
But lists can also be written in other ways that eliminate the ambiguity without introducing the serial comma, such as using other punctuation, or none, to introduce or delimit them. For example, in the following manner:
To God, Ayn Rand and my parents. Hemingway used and in place of commas in much of his work, and it was quite readable.
A famous example reportedly collected by Nielsen Hayden was found in a newspaper account of a documentary about Merle Haggard:
Among those interviewed were his two ex-wives, Kris Kristofferson and Robert Duvall. This could be taken to mean that Kris Kristofferson and Robert Duvall were Merle Haggard’s ex-wives.
Although Merle Haggard has been married five time, he was never married to either Kris Kristofferson or Robert Duvall, and a serial comma would resolve that inaccuracy:
Among those interviewed were his two ex-wives, Kris Kristofferson, and Robert Duvall.
I’m just going to say that argument is hogwash.
Who are your writing for, yourself or an unknown reader who may one day buy your book?
If you are writing for your own eyes only, do whatever you like.
But if you expect others to enjoy your work, you need to think about the reader: consider what is going to make your work easy for the reader to understand what you are saying.
Other aspects of commas may escape me at times, but the serial comma, also known as the Oxford comma, is one I adhere to in my own work, and heartily wish other authors would too.