You show your appreciation for the movie by paying twenty dollars to sit in a dark room on a still-warm chair and watch a movie.

You clap so that:

the projectionist can take a photo of the audience clapping,

then they post it on social media,

then a movie reviewer scans the social media posts,

then the reviewer mentions the photo of people clapping in a review,

then the review gets mentioned on a tv show,

then you can watch the tv show and tell your friends that you were one of the people clapping.