Why redirect? Why HTTP_REFERER?

Hello,

first: this post is not flame at all. I'm just curious.

I was thinking about web forum software in general
and it really seems to me that the best way for a web
forum is to use cookies. Because if you'd use hidden
fields, then you'd have to change every link into a button.
And another alternative, the URL: if you'd put the auth.
token into the URL, then you can't make bookmarks,
the URL also looks ugly and you can't send it to others.

So cookies are the way to go.

But why does PunBB shows a redirection page after
each action? If it's for setting cookies, then why can't
it just send the cookie-headers while displaying the next page?

And another question is: why HTTP_REFERER is needed
at all in PunBB. Since it is forgeable, as most here know.
What is it's purpose?

Thanks
Alex