Helping people do their work in an effective and pleasant way should be top priority in any design. If a product is not usable, people cannot work with it efficiently and consequently will be less productive.
Target-group test users should be involved at every stage in a design process. The best way to make sure that a product meets the target-group needs and expectations, is by having them interact with it.
They should also be involved after the product has been launched,
because the web site is alive, it grows and evolves with the advances
of business and changes in customer needs.