javascript, gulp, watch, changed

I cannot get my head around this. This is supposed to be a gulp task that is executed every time a watched file is modified. Can anyone explain why it is required to pipe the watched files through the changed plugin?

gulp.task('build-css', function () {
  return gulp.src(paths.css)
  .pipe(changed(paths.output, {extension: '.css'}))
  .pipe(gulp.dest(paths.output));
});

gulp.task('watch', ['serve'], function() { 
  gulp.watch(paths.css, ['build-css']); 
}); 

Disclaimer: this is not my code. Just trying to understand what is going on, in order to create my own custom watch task.