Piggy multiplies the amount of health you receive when eating fruit to 2x (20% rather than 10%), and turkey legs or hamburgers to 1.5x (75% rather than 50%). This is extremely useful for bosses which drop food, especially Necromancer. This ability does not apply to the popcorn created by the Corn Boss. Also, eating food with Piggy changes the sound to a pig-like snort.
While he's at the apex of his potential while fighting Necromancer, he's still very useful anywhere else, just not as much.