Why are progressCallbacks useful?

Question

A Deferred object has the callback pools doneCallbacks, failCallbacks and progressCallbacks.

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

What is a typical use case for progressCallbacks?

Answer 1

According to http://bugs.jquery.com/ticket/9398, it's offers progressive notifications. This is handy when dealing with long asynchronous tasks such as xhr2 uploads. So yes, as Guillaume86 suggests, it could be used in conjunction with a progress indicator such as a progressbar.