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!