It's all about the badges! You can get badges for giving people candy on Zilloween, the 3 recipes that count for those badges are Birch Candy, Hi-Sucrose Corn Syrup Squares, and Raw Bio-Organic Carob-ish Treats. (Please correct me if I'm wrong.) You can also get badges for eating said candy as well.
Edit: Most of the candy-swapping goes on in Cebarkul, or Somewhat Sump.