It's to make sure the buffer is flushed. Streams are closed, but not explicitly flushed at exit. Stdout only because it's the only filehandle with a buffer in yes.

In normal use, GNU `yes` does unbuffered IO on stdout. However, it does use buffered IO for --help and --version messages; it sets atexit(close_stdout) to cover both of those cases at once, rather than handling them both separately.