Page templates are used to determine the overall structure of a page.
The simplest template is a basic HTML page to be displayed when the page URL is requested. Whilst this may be useful for landing pages, most templates will include wrangler tags to insert dynamically generated content such as site menus on to the page, and regions for displaying articles.
Typically, a site will include several different templates: Any content pace directly into a template (such as a logo, or banner) will appear on all pages using that template. If you wish to add a banner only on certain pages, then the template may include a region to display articles of type "banner" - and it that case, only pages with a banner article attached to them will display the banner.