Happening sttatus: It's.
@Xoldin: it's just unspoken code. There is no "why" as such, it's essentially arbitrary like pretty much *everything* in communication, but once the meaning becomes widespread and accepted, it's commonplace and useful in communication. See: literally every instance of written words. Candy here is a symbol that seems to be accepted among teenagers as a signifier of romantic interest, and therefore Tadano including candy in his present is a roundabout way of saying "I would be interested" that is, nonetheless, relatively deniable ("I didn't know the significance, honest!") if Komi was to react with an "ewwww" (of course, we know this is impossible because they're head over heels for each other, but these two have self-esteem problems).