1. Congratulate someone else's achievement.
2. Acknowledge the end of a joke, speach or musical scene.
3. Sing the "Happy Birthday" song.
4. Force the speaker to shut the hell up.
5. Kill a flying insect.
6. Mimic a seal.
7. For the sake of peer pressure.
I'm just starting the applause so everyone can tell the joke is over and it's ok to laugh.
Dad: They're just giving applause to the movie they're watching.