我一直在查看HTML5样板模板，并注意到"?v=1"在引用CSS和Javascript文件时在URL中的使用。

1. "?v=1"在链接和脚本标记中附加到CSS和Javascript URL有什么作用？
2. 并非所有的Javascript URL都有"?v=1"（示例来自以下示例：）js/modernizr-1.5.min.js。有这种原因吗？

来自他们的样本index.html：

<!-- CSS : implied media="all" -->
<link rel="stylesheet" href="css/style.css?v=1">

<!-- For the less-enabled mobile browsers like Opera Mini -->
<link rel="stylesheet" media="handheld" href="css/handheld.css?v=1">

<!-- All JavaScript at the bottom, except for Modernizr which enables HTML5 elements & feature detects -->
<script src="js/modernizr-1.5.min.js"></script>

<!------ Some lines removed ------>

<script src="js/plugins.js?v=1"></script>
<script src="js/script.js?v=1"></script>

<!--[if lt IE 7 ]>
  <script src="js/dd_belatedpng.js?v=1"></script>
<![endif]-->


<!-- yui profiler and profileviewer - remove for production -->
<script src="js/profiling/yahoo-profiling.min.js?v=1"></script>
<script src="js/profiling/config.js?v=1"></script>
<!-- end profiling code -->