Why should we want more diverse protagonists in the narrative and visual arts? The argument I see most often focuses on the effect representation may have on the people represented. For example, it can be empowering to see yourself, your story, reflected in the narratives that we as a culture tell one another. A close second is the argument from realism: that diversity is a feature of society, and popular culture falsifies that reality to the extent that it tends toward a monolithic default protagonist.
Less commonly argued is the effect of representation on justice. I don't mean justice in the thematic sense — justice as a subject that an artist or storyteller may choose to explore in their work. Rather: justice as a matter of practice, as an aim of our activity in our work, our communities, and especially our courts. Historian Mary Frances Berry writes:
The interaction between stories and the interpretation of legal rules is a process. We change the law not by focusing exclusively on formal legal rules but by changing the experiences, and eroding the myths and stereotypes, that underlie each person's stories. […] If we want to insure justice we must give voice and power to previously silenced narratives, remembering that what the law does is a part of everyone's personal stories.
The Pig Farmer's Daughter and Other Tales of American Injustice, the colorful and engaging 1999 book from which that quote is drawn, presents a series of historical illustrations of her point. Each examines a different case from the annals of American jurisprudence, unfolding the ways in which the stories engrained in judge and jury clashed with the biographical context of the participants own lives, to the detriment of the cause of justice. Berry's overarching point is that changing the legal process is often not enough to ensure just outcomes. Since familiar narratives shape how we interpret both the facts of a specific case and the law in general, systemic change also requires expanding the store of narratives that the culture at large makes available to us.
If the practical import of that seems a bit airy and abstract, I recommend Berry's book. She gives the subject significant heft and solidity. My point here is only that diverse representation in popular culture can have the effect of expanding the breadth of stories we bring with us into considerations of justice. It does so not only by helping us imagine scenarios outside our individual experience, but also by helping us visualize different sorts of people in those scenarios.
If that were less effective at opening the possibility of justice for the oppressed, then their oppressors likely would not be so outraged each time popular culture threatens to become the slightest bit more diverse.