Without an Oxford comma, meanings of sentences can change completely. For example:
Phil found himself in the Winnebago with his ex-girlfriend, an herbalist and a pet detective.
vs.
Phil found himself in the Winnebago with his ex-girlfriend, an herbalist, and a pet detective.
One comma makes the difference between a uncomfortable road trip with two people (one of whom is an herbalist and pet detective) and a trip with four people!
