Only satisfied users will use that site and come back. Usability means quality of interaction. Product developers more and more understand, that the user must play the most important role in the development of a system. Not the user should have to adapt to a system but the system should be designed to fit the needs of the user. It should be easy for each user to use a system.
Usability is an important criterion in competition.
Especially web systems should be of high quality because of the many user groups of the system and because for the webuser, the website IS the organisation.