First of all, the purpose of a template is to control the look of your site and make your life much easier when creating new pages. Joomla is designed to separate out the key tasks involved in producing a website. One of these tasks is to create the look, feel and layout of the site. This includes making decisions such as which content elements such as components, modules and plugins you may want to place in any given page.
When producing a web page, the location of most elements will stay the same (menus, banner locations, sidebars etc.). Additionally, you will want to create the same look (fonts, header styles, color scheme etc.) for every page. For some parts of the site you may want to alter the general feel to indicate a different purpose for those pages (such as a blog section).
And that’s where the template comes in. You can either write all the code for each page separately, or use a template for each major section of your site so that when you want to create a new page you simply “fill in the blanks”.
A template can be designed to alter how it displays on different web browsers, allowing you to take full advantage of the latest developments without making your site inaccessible to those who are not able to run "up-to-the-minute" system upgrades (such as certain companies who limit what software their employees can use).