Better usability means a better experience for the visitors to your site. And this translates directly to better sales and service.
Fulfilled visitors are much more likely to make purchases.
If you offer support services, improved usability delivers tangible benefits with fewer calls to your support desk. This applies equally whether you run a multinational corporation, or a small family mail-order business. Having a usable site gives you credibility in the global marketplace of the web - after all, you surely want to look like an efficient organisation rather than an amateurish outfit. Stand out from the crowd and at the same time increase your business.