Template Hierarchy System
The WordPress template hierarchy system allows developers to create individual designs/templates for each page. There is a specific order to how WordPress will choose the template file, but in general it is important to understand that this system is in place and not all pages have to have identical designs. WordPress will automatically look for a template file based on the file's name.
Template Hierarchy System Order
If the page has a custom template assigned in WordPress, it will use that template. Otherwise, WordPress will automatically pull a template file based on its name. First, it will look for a file that matches the page's slug. If that doesn't exist, it will look for a file with the page ID. If neither of those files are present, it will continue on to look for the theme's default page template, then for an singular.php file, and eventually it will fall back to the main theme file of index.php.
File Naming Conventions
The file naming conventions are important when setting up these specialized theme files. They must match either the slug or page ID. For example, a page with the name of "cat" and ID of 37 would require a specialized theme file named either page-cat.php or page-37.php. Note that these specialized templates are separated with a hyphen; to create a custom template that will be assigned in WordPress, you should use an underscore (page_cat.php).
Summary
It is important to understand the WordPress template hierarchy system to properly style each page according to design needs. Without this system, all of the pages would have the same style, which wouldn't be suitable for many sites' needs.