Boredom. Here is the student, trying to sit still, trying to look only at the teacher or at one's own notes, trying not to be distracted by other students' actions. Annoyed when the class is reviewing material one already knows, or completely at sea when the lesson content makes no sense, and yet if one asks too many questions, the other students will think one is dumb. When will the class period end?
Then -- hooray -- it's time to play a game. Everyone gets out of their desks, moves around, is able to interact and express personality, is allowed to enjoy being human.
Home that evening. What did you do at school today? We played a game.
A year later: Did you learn this material? Yes, we played a game, it went like this. Smiles, remembers, enjoys telling how to play the game. Wants to play it again.
A fun classroom game is not only exciting for teacher and students, it is highly effective for learning!!