It's a grammatical rule to use the Oxford comma. I think most teachers these days say it's optional, but there's a big difference between "I took my dogs, Mark and Peter, to the park," and "I took my dogs, Mark, and Peter to the park."
What comes up when you search for the oxford comma: "The 'Oxford comma' is an optional comma before the word 'and' at the end of a list: We sell books, videos, and magazines. It's known as the Oxford comma because it was traditionally used by printers, readers, and editors at Oxford University Press."
When people don't use it I freak out.
"The 'Oxford comma' is an optional comma before the word 'and' at the end of a list: We sell books, videos, and magazines. It's known as the Oxford comma because it was traditionally used by printers, readers, and editors at Oxford University Press."