This filter can be used by plugin that offer CSS pre-processors, to store the original pre-processed CSS in post_content_filtered and then store processed CSS in post_content.
When used in this way, the post_content_filtered should be supplied as the setting value instead of post_content via a the customize_value_custom_css filter, for example:

add_filter( 'customize_value_custom_css', function( $value, $setting ) {
    $post = wp_get_custom_css_post( $setting->stylesheet );
    if ( $post && ! empty( $post->post_content_filtered ) ) {
        $css = $post->post_content_filtered;
    }
    return $css;
}, 10, 2 );