Here's where it gets interesting! The Form Page Template Type is used to generate the HTML used for your form pages. So, if you published a form that arranged the fields into three View tabs, those tabs are converted into form pages.
This page needs to contain the (relatively complex) Smarty logic needed to loop through each field group and display the page title (the View tab label), the headings for the field groups (the View Field Group label) and the fields themselves.
It also needs to contain the navigation template (to show users how they are progressing through the form) and the Continue Block template, to allow them to proceed to the next page.
Lastly, it needs to wrap everything in a form tag so that the data is submitted to the server when the user clicks continue.
Here's an example taken from the Default Template Set.