Starting with version CE-1.6 (I think) there is a new module added to Magento.
This offers a "Remember me" feature when you log in. If this is enabled a cookie is set and the user is allowed to access some personal data (cart, wishlist) without the need to log in.
I don't know why this approach was taken. It would have been easier just to include this functionality directly in the
Maybe this was intended at first to be available only in Enterprise and that's why is a separate module, but I'm just speculating.
The idea is that this module replaces the templates of some blocks in order to make the
Remember me functionality available.
The replaced templates are the customer login and register and the checkout login and billing steps.
To see what and how is replaced check
/app/design/frontend/base/default/layout/persistent.xml layout file.