!!! Overview
[{$pagename}] as [HTTP] is [stateless], in order to associate a request to any other request, there could be a need to store [{$pagename}] between [HTTP Requests].

[Cookies] or [URL] parameters ( for ex. like http://example.com/myPage?asd=lol&boo=no ) are both ways to transport data between 2 or more [HTTP Requests]. 

So even though the [HTTP] [protocol] may be [stateless] the long-term communications between a [HTTP] client and [HTTP] [server] are NOT.

If properly implemented, [{$pagename}] are not good in the [use case] you do __NOT__ want that data to be readable/editable on [client-side].

The [Best Practices] solution is to store that data [server-side] and only pass the [{$pagename}] [by-reference] and client only knows that reference id and that it must pass the [{$pagename}] to the [server] with each request. 

Of course there are other aspects to consider, like you don't want people to [hijack] other's [sessions], you want [sessions] to not last forever but to expire, and so on.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]