It separates the presentation styles and template files from the system files so the site will upgrade without drastic changes to the visual presentation of the site.
It 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.
It’s an opportunity to put your expertise with CSS, HTML, and PHP to work.
It’s fun (most of the time).
If you release it to the public, you can feel good that you shared and gave something back to the WordPress Community (okay, bragging rights)