What are the advantages of battery chickens?

From the chickens point of view there are few. Perhaps the only positive is they are not subject to the "pecking order" in a chicken coop.

For the human, the battery operation is cost effective. Huge supply of eggs, ease of maintenance, less staff involved in general care. Assembly line style of feeding, egg collection. Lower costs of production equates to lower prices for consumers and therefore higher demand for the product resulting in higher profit margins.