When I first saw The Force Awakens, I went with a friend, and we had so much fun talking about the movie afterwards that we managed to miss the right exit on the highway twice. One of my observations was the large number of female background characters present in the movie. She admonished me for not saying that about the protagonists.
Granted, that's generally the more important category to be concerned about, but here's why attention to extras matters too--and why a change in their demographics is so striking.
A good protagonist always stands somewhat apart from the world in which they live. It's important to have heroes that represent many demographics, but upon gaining protagonist status, a character becomes privileged in the eyes of the audience. It seems obvious to us that heroes are extraordinary people, so we might not notice that the women, people of color, etc. have to be protagonist-level-awesome just to get a place on screen at all.
It's the background characters that define what the rest of the world looks like: what the characters themselves consider to be "normal". Well-chosen extras prevent a diverse batch of heroes from seeming like a "band of tokens", because it's obvious that the heroes aren't the only ones of their respective groups who get to participate.
Consider Princess Leia in the Original Trilogy. She's the only woman with any large speaking role in that whole part of the story. The reality of her world is that good guys and bad guys alike are exactly that--guys--but hey, if Princess Leia says she's coming on the mission too, no one's going to argue with her. It's a man's galaxy: Leia needs Princess status to get in, and maintains her place not just by being as awesome as she is, but also by being the male lead's sister, the other male lead's love interest, the bad guy's daughter, and the Backup Jedi.
Contrast this with the extras we see in The Force Awakens. There are lots of women and people of color in background roles, and what this says about the world is that it is now considered normal for a wide variety of people to participate in the action. It's awesome that The Next Jedi is going to be a woman, but we can also see that a woman doesn't have to be The Next Jedi just to earn a place on screen, while men can get their places by being nameless pilots.
Honestly, as cool as it is, this kind of change is so powerful that the whole setting looks fundamentally different for it. It looks as though the entire galaxy had a massive equality movement between trilogies, whose victory was so rapidly and overwhelmingly successful--more so than the original Rebellion!--that even the First Order is an equal-opportunity employer (at least, as long as you're human).
Some might say that the diversity in this movie is, in part, a sign of the time at which it was made, just like the quality and style of its special effects. If it looks jarring that the galaxy had a sudden equality shift, you might as well wonder why it had a "suddenly the lightsaber blades look better" shift. That's not quite right, though, because movies are made by people, and while everyone agrees that working on better special effects is a good idea, not everyone realizes or cares that diversity in casting is a good idea too. Many movies made at the same time as TFA were not this diverse, because people chose to not make them diverse.
Every background character is the result of a choice someone made to put them there. When the extras are diverse, it means someone made the right choice not just a few times for the heroes, but many more times for the rest of the cast too. More people need to be making these choices. I don't want "diverse extras" to become a distinctive trait of just this one universe. Star Wars has enough exclusive recurring themes already; it needs to share this one.
P.S. Genre-savvy readers will note that Star Wars has a long history of bestowing a name and full life story upon every single background character through the Expanded Universe. That means that all these extras have a good shot at becoming protagonists themselves someday. It makes these casting choices all the more important: they affect the Expanded Universe cast for decades to come.