I was reading up on how medieval battles were actually fought in a book by John Keegan called "The face of battle". In one of the introductory chapters, he mentions the writings of Julius Caesar on his campaigns in Gaul. Keegan quotes a part in which Caesar reinforces a flank that is on the verge of collapse. He mentions that "some were leaving the battle to join those in the rear ranks who were already making oil."

What was the purpose of the making of oil? I was thinking about setting the baggage train on fire, or even an entire field if possible to minimize the routing? Was it for some other purpose? Please, enlighten me.

edit: Okay, so the first reply made me realize that I forgot a part of my question. I understand the advantages of fire. Why specifically during a retreat? How big is a Roman battlefield, is it better measured in football fields or in miles?