“On the Web, usability is a necessary condition for survival. If a website is difficult to use, people leave. If the homepage fails to clearly state what a company offers and what users can do on the site, people leave. If users get lost on a website, they leave. If a website’s information is hard to read or doesn’t answer users’ key questions, they leave. Note a pattern here?” – Jakob Nielsen
Read Jakob’s take on why usability is important in his article “Usability 101: Definition and Fundamentals – What, Why, How” here.