Grammar, the main difference between 'Let's eat, Grandma!' and 'Let's eat Grandma!'
So technically it's not incorrect to omit the comma, though it does define a cadence and provide clarity.I’m not an expert, but writing conventions do not mimic the cadence of spoken language. Instead they help readers decode words and phrases. This is an excellent example. In print, the comma helps the reader visually parse what was said, while distinguishing to whom it was said. In a novel, this occurs in almost every paragraph. Quotation marks and commas are used as delimiters so that the reader can keep track of who is speaking and listening.