“User success rates on e-commerce sites are only 56%, and most sites comply with only a third of documented usability guidelines. Given this, improving a site’s usability can substantially increase both sales and a site’s odds of survival.” From Did Poor Usability Kill E-Commerce? by Jakob Nielsen.
He also writes that “E-commerce sites lose almost half of their potential sales because users cannot use the site. In other words, with better usability, the average site could increase its current sales by 79% (calculated as the 44% of potential sales relative to the 56% of cases in which users currently succeed).”
Good web design is all about achieving your goals