A Drupal website has three main components: the nuts and bolts (functionality), the actual content, and the theme layer (or skin) of the site.
The beautiful thing about this system is you can change the look of your website without having to lose the nuts and bolts or the content. It stays, the skin just changes. You might just want to freshen your theme every year or so (to match a new product?) or change seasonally... or even change by the time of the day or the type of visitor. Your options are endless.
A key thing that many designers overlook is that your Drupal theme can impact how you score with SEO. Think of your theme as a lens for not just people and how they see you, but for search engine spiders as well.
Your Drupal theme needs to validate for proper formatting too (CSS, XHTML, RSS, etc.) so that it is compatible with multiple browsers and mobile devices. Getting your site to look beautiful for everybody is a tricky business, especially because of Microsoft's Internet Explorer (the misery of web developers everywhere.) Superclean goes through a lot of effort to insure that cool effects such as drop-down menus and "transparent" graphics work for everybody.