A WordPress Theme is a collection of files that work together to produce a graphical interface with an underlying unifying design for a weblog. These files are called template files. A Theme modifies the way the site is displayed, without modifying the underlying software.
Custom Page Templates allows for customization of the site functionality unique to that Theme. It allows for quick changes of the visual design and layout of a WordPress site. It removes the need for a typical WordPress site owner to have to learn CSS, HTML, and PHP in order to have a great-looking website.