Ruby on Rails: alias_method_chain, what exactly does it do?

I've tried reading through various blog posts that attempt to explain alias_method_chain and the reasons to use it and not use it. In particular, I took heed to:

http://weblog.rubyonrails.org/2006/4/26/new-in-rails-module-alias_method_chain

and

http://yehudakatz.com/2009/03/06/alias_method_chain-in-models/

I still do not see any practical use for alias_method_chain. Would anyone be able to explain a few things.

1 - is it still used at all?
2 - when would you use alias_method_chain and why?