The Drupal Theme Layer
A Drupal CMS website has three main components: the body (files and scripts it takes to run it), the brain (database with content), and the skin (theme layer.)
The beautiful thing about this system is you can change the appearance of your website without losing or misplacing your body or brain. You might freshen your theme every year or so (to coordinate a new product release?) or change seasonally... or even change skin dynamically by the time of the day or the IP address of the visitor. Opportunities are endless.
Your Drupal theme also impacts your SEO tremendously. Think of your theme as a lens for not just people and how they see you, but for search engine spiders as well. The algo's are judgey about how your pages render.
But the most important thing is: user experience. Especially on mobile. And it shouldn't *just work* across all devices. It should be a joy to use!