We might divide a page template into a header, content, sidebar and footer where these are in separate files. The content will change most between pages, so rather than having a content file, we include the content into a page file which pulls in the individual template files and defines the unique features of that page. The unique features of a page will be the title, description and article content. We can define these at the start of the page template with say PHP code to set the value of variables that are used by the other templates such as the header template. Then we assemble the page by including the required templates and inserting the content.
In the old days people would always hire a professional website designer because they felt this was the best way to ensure they got a professional looking website made. Companies and individuals were willing to spend thousands of dollars for the chance to create a web presence that would give them a good image. Nowadays, especially because of the state of the economy, many companies and individuals are deciding to go with website templates instead of a website designer or developer. There are a multitude of reasons for this.