Costume Quest 2 is Double Fine's first sequel, and the one that, according to Greg Rice, was "probably the most requested besides Psychonauts." What fans wanted was more of the same, and that's what they're getting, right down to Reynold's Candy Corn costume, which was how the whole mess got started in the first Costume Quest. It wasn't a usable costume then, but it is now. A completely useless, utterly awesome costume.
The gimmick for the turn-based combat in Costume Quest 2 is that the trick-or-treating heroes can swap out Halloween costumes, thus granting themselves different attacks. The clown, for example, has a special move called Laughter Is the Best Medicine which heals the party, while the superhero costume comes with devastating uppercuts. The candy corn ... sits there. (It's just a big piece of candy corn, what did you really expect it to do?) It's a hilarious running gag in combat, noted each turn by phrases like "Candy corn is doing calculations" and "Candy corn chooses not to participate."