I was working on a site that encodes cookies today and I was wondering why they did that. I was thinking that "hey, encoding is not the same as encrypting...are they doing this for security reasons?" Then I started thinking about it a little more - the distinction between encrypting and encoding. I did a quick search which provided a nice document that I am giving kudos to for backing up my thoughts on the technically correct purposes of encode and encrypt.
http://www.di-mgt.com.au/encode_encrypt.html