The key to this emoji is the "eye smile." In Korea especially, an eye smile is seen as a charming and sincere expression. That's why this emoji is popular for showing unadorned, pure joy.

Just as the "eye smile" of many K-pop idols is a huge charm point for fans, this emoji symbolizes happiness that comes from the heart. It therefore feels warmer and more human than other smiling emojis. Use it to sincerely congratulate a friend or express gratitude, and your feelings will come across clearly.