At work, we had a speaker who talked about his company's diversity hiring approach. I don't recall specifics, but I believe it was a bank that was pretty well-known for having good representation across race and gender -- which was the basis for the invitation to come speak.

The bit that stood out for me was the claim that they never hired merely to increase diversity. Instead, they hired only based on the merit of applicants.

To achieve diversity, however, they were very aggressive about inviting minority applicants to apply, for example, posting job notices in places where under-represented groups were likely to see the notices.

Applying that idea to OSS, one could imagine active outreach as a means of increasing diversity among contributors.

The larger question of why one should care about having diverse contributors has, I think, been largely addressed by other commenters already. In addition to the UI issues others have mentioned, I might also guess that, from the perspective of Free Software Foundation, they probably also want to understand and engage minorities' privacy concerns.