-The show is about Maarva Andor. That's the twist, that the whole time we think it's about Cassian, even though literally everyone in the show is just saying he's a pawn but he happens to be a pawn in the middle of the board.
-Maarva Andor just started the Rebellion proper. Not a terrorist group. Not a squad of soldiers attacking an outpost. Normal citizens standing up against Imperial oppression. This is it, this is what the whole series was building towards.
-Take any one element away and it doesn't happen. Maarva needs to meet Cassian and start caring. She has to promote him to Luthen, and Luthen has to fight to recruit him and accidentally push too hard. They have to do the heist, including with the selfish guy and the philosophical guy. Mon Mothma needs to supply the money for that group. The ISB agents need to be tracking Andor to Ferrix. And so on and so on. All the disparate pieces that have been covered in loosely arranged "arcs" beforehand, all of those pieces needed to come together to finally get everyone on the brink of open rebellion, and Maarva Andor's speech was what sparked the fire.
-Not only that, but her sacrifice, and what the Empire did to Ferrix, to Bix, to the prison, that's what had to get Andor to where he'd actually give everything, all at once, to fight the Empire. So the finale sees him join the Rebellion in earnest, a true believer now. With Luthen as the spymaster who godfathered it, and Mon Mothma as the mother who will nurture it from here, having sacrificed her own child to caretake this one.