Why are progressCallbacks useful?
<p>A <code>Deferred</code> object has the callback pools <code>doneCallbacks</code>, <code>failCallbacks</code> and <code>progressCallbacks</code>. </p>

<p>Both <code>doneCallbacks</code> and <code>failCallbacks</code> (together with the methods <code>done</code>, <code>fail</code>, <code>always</code>, <code>reject</code>, <code>resolve</code>, etc.) are useful.</p>

<p>What is a typical use case for <code>progressCallbacks</code>?</p>
