Breq isn't a Chosen One. She isn't The Most Important anything, and the entire Radch isn't looking to her as either their greatest enemy or their greatest hope. She isn't the most legally or socially powerful person, but arguably neither is she the most hated and oppressed (she's considered equipment, which is pretty bad, but all the people killed in annexations or effectively enslaved on the tea plantations have a decent argument that they're worse off, and we see her realize in Mercy that her new situation has placed her enough above her ship that it's affecting her treatment and views of a person like herself). She isn't quite an insider, realizing her society is broken, but she quite isn't an outsider seeing all the problems the way an outsider would; she's technically part of the Radch, has spent most of her life working within Radch culture, but the culture was never created by or for her. She does technically have a "secret heritage" which changes how others view her and which affects her social and legal position, but it isn't secret from her or the readers, so we experience it not as a shocking plot twist but as a person hiding the socially unacceptable parts of her identity from society at large so she can get by. She doesn't accomplish most of her work in exciting fights, but by listening to people's complaints and organizing maintenance repairs: the slog of everyday work.
In short, she's a lot closer to your average person than many characters in this sort of story often are. Most of us aren't, never will be, and never expect to be the supreme leaders of any organization or movement we join, and most of us aren't at the absolute bottom rung either--we're usually at some point in that fuzzy middle section. Maybe on the lower end of the scale, like Breq. And maybe whatever power we might acquire is on a smaller scale, the way Breq gets a ship and a tiny corner of space. Most of us aren't complete outsiders to the societies we live in, but many of us aren't the people our societies value most or benefit most (to varying degrees), so we can understand Breq's double perspective, both inside and outside simultaneously. Most of us spend a lot of our time slogging through cleaning and organizing and fixing-type work, over and over again.
And these books tell us that average doesn't mean insignificant. It doesn't mean useless or powerless. It doesn't mean boring. Breq accomplishes a lot without being Chosen or the Most Important Whatever.
Most of her accomplishments start small, and we see the effects on individuals' lives. The impoverished people who now have fresh air in their homes. The person who isn't wrongfully convicted of a crime just for being a different ethnicity than the police. The teenager rescued from an abusive boss. The other teenager freed from that same abusive person as significant other.
And these small accomplishments add up, and promise larger changes in the future. Not completely and instantly--we don't expect this entire society to suddenly become equal and cruelty-free in a few weeks--but we see enough to feel that Breq has made things better in this tiny corner of space, and that they can continue to get better. Her example may even inspire change elsewhere in the Radch.
This is the kind of change that feels achievable to me, as a reader. Most of us probably don't see becoming the leaders of our entire societies and radically transforming them as a possibility. But we can imagine helping, say, our neighborhoods, or changing a few lives through our jobs or volunteer work, or maybe slowly changing minds after dozens and dozens of conversations with people who mostly don't want to listen. So in many ways, I actually find this more inspiring and hopeful.
Which isn't to say that I never find the stories about those all-powerful rebel leaders inspiring or hopeful! It's just that after reading lots of them, a story about a (relatively) average person who does a lot of scut-work and it matters is the story I was missing and desperately needed.
You don't have to be the best or the most powerful or the center of attention to matter, or to make a difference: that's what these books say to me. And I love them for it.