Why Zen theme uses these?

Zeta, you are too quick and concise! Thanks.

I am posting this incase someone else also wants to know this. The following function drupal_get_css adds the dummy query string to the css files. The same way drupal_get_js adds the dummy query string to js files. Both these function reside in includes/common.php

function drupal_get_css($css = NULL) {
  $output = '';
  if (!isset($css)) {
    $css = drupal_add_css();
  }
  $no_module_preprocess = '';
  $no_theme_preprocess = '';

  $preprocess_css = (variable_get('preprocess_css', FALSE) && (!defined('MAINTENANCE_MODE') || MAINTENANCE_MODE != 'update'));
  $directory = file_directory_path();
  $is_writable = is_dir($directory) && is_writable($directory) && (variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC) == FILE_DOWNLOADS_PUBLIC);

  // A dummy query-string is added to filenames, to gain control over
  // browser-caching. The string changes on every update or full cache
  // flush, forcing browsers to load a new copy of the files, as the
  // URL changed.
  $query_string = '?'. substr(variable_get('css_js_query_string', '0'), 0, 1);

  foreach ($css as $media => $types) {
    // If CSS preprocessing is off, we still need to output the styles.
    // Additionally, go through any remaining styles if CSS preprocessing is on and output the non-cached ones.
    foreach ($types as $type => $files) {
      if ($type == 'module') {
        // Setup theme overrides for module styles.
        $theme_styles = array();
        foreach (array_keys($css[$media]['theme']) as $theme_style) {
          $theme_styles[] = basename($theme_style);
        }
      }
      foreach ($types[$type] as $file => $preprocess) {
        // If the theme supplies its own style using the name of the module style, skip its inclusion.
        // This includes any RTL styles associated with its main LTR counterpart.
        if ($type == 'module' && in_array(str_replace('-rtl.css', '.css', basename($file)), $theme_styles)) {
          // Unset the file to prevent its inclusion when CSS aggregation is enabled.
          unset($types[$type][$file]);
          continue;
        }
        // Only include the stylesheet if it exists.
        if (file_exists($file)) {
          if (!$preprocess || !($is_writable && $preprocess_css)) {
            // If a CSS file is not to be preprocessed and it's a module CSS file, it needs to *always* appear at the *top*,
            // regardless of whether preprocessing is on or off.
            if (!$preprocess && $type == 'module') {
              $no_module_preprocess .= '<link type="text/css" rel="stylesheet" media="'. $media .'" href="'. base_path() . $file . $query_string .'" />'."\n";
            }
            // If a CSS file is not to be preprocessed and it's a theme CSS file, it needs to *always* appear at the *bottom*,
            // regardless of whether preprocessing is on or off.
            else if (!$preprocess && $type == 'theme') {
              $no_theme_preprocess .= '<link type="text/css" rel="stylesheet" media="'. $media .'" href="'. base_path() . $file . $query_string .'" />'."\n";
            }
            else {
              $output .= '<link type="text/css" rel="stylesheet" media="'. $media .'" href="'. base_path() . $file . $query_string .'" />'."\n";
            }
          }
        }
      }
    }

    if ($is_writable && $preprocess_css) {
      $filename = md5(serialize($types) . $query_string) .'.css';
      $preprocess_file = drupal_build_css_cache($types, $filename);
      $output .= '<link type="text/css" rel="stylesheet" media="'. $media .'" href="'. base_path() . $preprocess_file .'" />'."\n";
    }
  }

  return $no_module_preprocess . $output . $no_theme_preprocess;
}

Loving Life,
Tosho Freny,