Partials make large websites easier to maintain, they are like a function, they are HTML that can be reused on different pages. Nav bars and Footers are a common example of partials.
<div class="header clearfix">
<nav>
<ul class="nav nav-pills pull-right">
<li role="presentation"><a href="/">Home</a></li>
</ul>
<h3 class="text-muted">Node.js Blog</h3>
</nav>
</div>
<footer class="footer">
<p>© 90210 Lawyer Stuff.</p>
</footer>
<body>
<div class="container">
<%- include('partials/navbar') %>
<div class="jumbotron">
<h1>All about Node</h1>
<p class="lead">Check out our articles below!</p>
</div>
<%- include('partials/footer') %>
</div>
</body>