I recently attended a talk by Bill McNabb, CEO of Vanguard. Vanguard promotes gender diversity internally and at companies they own shares in. I thought the rationale he described was eloquent and applicable in general. Three points stood out.
First, the goal is corporate performance. Diversity provides material value.
Second, the probability of having all effective leaders in a group partitioned by gender is lower than in an unpartitioned group.
Third, research findings (example) support the hypothesis that a diverse board yields higher performance.