My favorite types of yogurt are orange creme, pineapple, strawberry and banana and vanilla.Are there three flavors? Four? Two? Is strawberry and banana one flavor or is banana and vanilla one flavor? It's not clear. It's also awkward because the reader doesn't know when to finish the sentence. Now consider the same sentence with one more comma.
My favorite types of yogurt are orange creme, pineapple, strawberry and banana, and vanilla.Much clearer, isn't it? A friend of mine once told me that commas separate each idea in a sentence as if they were the gaps between cars of a train. It's a pause to signal a slight change in thought. Don't treat it like a period (i.e. watch sentence length) but use it right in a list.