The two main parts of a template are the content management code and the theme. These should be kept separate so that the theme can easily be changed without affecting the structure. For example, the styles should be defined in an external style sheet and not embedded in the HTML code of the pages. The structure of a website will be defined by a set of templates. It is good practise to make the templates modular rather than having page templates containing all the code for each page. This allows site-wide changes to be easily applied. For example, the footer may be mostly identical between pages.
I would say the number one benefit to website templates is that they‘re going to save you from having to wait for somebody else to get done with your website. You won‘t control don‘t you? Well website templates allow you to have a personal website set up within a few days after purchasing. All you would need is to select the platform you want and then choose templates that are going to suit it. Now compare this to trying to do everything yourself or hiring a professional web developer. It can take days before the product is even ready to get started.