The "Template:" feature allows you to decide what website template you would to use for the page. Each page can have its own look and feel independent of the main website. The default template is the template that you have set in the web builder config menu; possibly the template you selected during the initial building process.
*Note:The "Template" section is similar to the "Add to Menu" section in that it allows you to customize each page differently. You can use both the "Add to Menu" and the "Template" sections in correlation with each other. For example: I like to change my site's look and feel for the holidays especially Christmas. So, I created new pages with a Christmas template and save them to a new menu that I titled "Christmas". Then every year on December 1st I change my menu to the Christmas menu and activate those pages. So every year it's easy to put some Christmas Spirit into my website.