Rails - Nov 2014 - What is a purpose of Rails.root/tmp/sessions ?

When I read rails guide 
<http://guides.rubyonrails.org/command_line.html#tmp> ,  I meet this 
sentence `rake tmp:sessions:clear clears tmp/sessions.`.
In Rails maybe there are any options to store sessions data to File 
(default session store is CookieStore).

Dose anyone know a purpose of this directory?
Thank you.

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