It's most commonly referred to as the serial comma and "generally" it's avoided in journalism and marketing circles, and used everywhere else. The question usually only comes up on boards like this though. The argument each way is that using it can introduce ambiguity, and the rest of the world just rewrites the damn sentence to make sure it doesn't happen.![]()
It depends on the style guide that you're following. I work as a contractor for the US Federal Government (DoD Contractor to be exact) and the US Government Printing Office Style Guide requires (yes, they use the word 'require') the use of the serial comma for all lists of three or more items when the conjunction is 'and' or 'or'.I always thought that it was used in the States and avoided in Ireland and Britain. I was taught in school (in Ireland) not to use a comma before "and" except in exceptional circumstances. I'm doing a journalism master's now and we are taught to avoid it.
I should have mentioned that "egg and tomato" is one dish. Anyone who didn't know that would not know which are the two or three dishes:
"snow peas, and egg, and tomato" or
"snow peas and egg, and tomato" or
"snow peas, and egg and tomato"