Io's the first one Akira opens up about Sekai with. And I'd point out that even more impactful than he saying that he loves Sekai is that he recognizes that he shoudn't, and that it's probably stockholm's.
It's important because it states the dissonance in his mind. He knows logically that he should see her as an enemy, and yet he can't break free of her infection.
Io stating that she'd kill her mother is way more advanced in terms of dealing with it than Akira's (granted, they are very different scenarios). But I think, to an extent, it also represents the difference that it made for Io to receive counseling and a support group.
Ultimately, Akira needs to brink Sekai into the light so that he can properly exorcise her. Which I think is why Himawari is present in her restoration. They summoned the ghost, so now it can be defeated once and for all (I hope).