There are a few major reasons advanced for this:
1. Economically speaking, companies that do not diversify rob themselves of the potential talents of people from the ethnicities (or other groups) that they do not hire.
2. A variety of viewpoints (believed to come in part from diversity) can improve a company's ability to innovate.
3. Diversity can help a company market itself to a larger variety of communities whether that be ethnic groups within a country or people in foreign countries. This point of view argues that minorities and/or foreigners will feel more comfortable dealing with companies who have "people like them."