Need for custom tags in JSF and JSTL/El

Question

Why do we need to use custom tags in JSF and JSTL/EL even though there are sufficient in-built tags (HTML or otherwise) are provided? What are the applications and usages of them?

Answer 1

A little late, but this might be useful for your question: http://blog.oio.de/2012/05/23/custom-tags-as-an-alternative-to-composite-components-in-jsf/

Tags basically can act as placeholders for larger snippets of code. You might for example use a simple tag like for a larger construct like

<div class="outer">
    <div class="inner">
        <div class="headline"> #{headline} </div>
        #{content}
    </div>
</div>

You could easily use this like

<panel headline="My Headline" content="My Content" /> 

Dont confuse custom tags with composite component, btw. Those are even more powerful than custom tags.