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

When I read rails guide , 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.

Prior to the cookiestore, the default rails session store was filestore, which uses ruby’s pstore mechanism. It was never really suitable for use other than development, since it presumes that all of your app instances have access to this shared storage.