I think it's an acknowledgement that race and gender are self defined and often don't fit what society defines them to be. Like a Black person in the US may not consider themselves African American because they identify more as Afro-Latino or Carribbean, but society would still identify them as African American. Or for gender, people who are gender non-conforming are oftentimes misgendered by society as a whole.

Now you could make the argument that gender and race should already mean self definition rather than societal definition. But we still live in a world that has a tonne of weird race science and transphobia. So using the word "identity" I think recognizes the more important aspects of race and gender.