Users have more satisfaction and are more likely to return to a site when they have a good experience using it. When an interface is easy to use, it decreases the amount of time it takes to learn to use it. Usable interface solutions improve productivity, minimize risk, and reduce cost.

Early detection of usability issues through prototypes and user testing reduce costs in enterprise level projects because the issues can be resolved before development. There have been examples where by just changing the wording on a submit button a substantial increase in revenue has resulted, because the new wording increased the likely-hood of the user to fill out the form. Usability also reduces the number of calls to customer support, which saves money after release.

Good usability practices reduce the occurrence of exceptions or errors because you don’t put the user in the position to create errors. If an error does occur it should be easy to recover from. Creating an elegant interface that is easy can will create user trust and loyalty, and increase the number of times they are likely to return to the site as it creates a more memorable experience.

Usability rules are general guidelines that can be followed during development to keep from making usability mistakes. They can also that can be used in a heuristic evaluation to check or compliance after development.

Example Usability Guidelines
http://www.usability.gov/guidelines/