What version of Drupal is your site running on? Usage statistics of Drupal Core say that some still use Drupal 6 and 7. There are even websites that are still working on Drupal 5. Using old versions, you deprive yourself of the new features of the admin panel and community-supported modules, themes, and profiles. Your customers are left without a modern user experience and cannot be sure your website is secure. Support for these versions is over.
One of the characteristics of a modern website is accessibility which means that anyone, including people with disabilities, can use your website without any problems. WHO estimates that 1.3 billion people, or one in six people worldwide, have a significant disability. This is a massive audience for your website. They will purchase your products and services if you make your WCAG Compliance website available to everyone.