Rails - Jul 2014 - What is the actual use of ActiveRecord::SpawnMethods#except ?

I really don't understand the actual usecase of the *except* method 
http://api.rubyonrails.org/classes/ActiveRecord/SpawnMethods.html#method-i-except

If I don't want a condition to apply in my _query_, then why should I add
it?
After adding a condition, what is the point of removing it again. This how the 
doco
http://api.rubyonrails.org/classes/ActiveRecord/SpawnMethods.html#method-i-except
is saying.

-- 
===============Regards,
Arup Rakshit
===============Debugging is twice as hard as writing the code in the first
place. Therefore,
if you write the code as cleverly as possible, you are, by definition, not 
smart enough to debug it.

--Brian Kernighan

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