The immediate hash (e.g., cTIwP01Sm6wbO-co1YnspWwpmujJAfZPx-cq4PHUbpc%5B*/ above) is a hash code that is generated from the Access Group and the user's ruleset profile. Are you referring to this, or the pzPostData?
Here's the answer I got from one of my colleagues:
When we POST data to PRPC we have to handle the case when a redirect occurs - for example when posting data to just the /prweb/PRServlet will cause a 303 redirect so the processing runs within the proper context - accessgroup hash
so we need to store the posted data because a 303 redirect response will then trigger a GET request from the browser
so the pzPostData is a pointer to a hashmap containing the posted data so it can be referenced/used when redirects occur.