[Klashikari]

Why is there a need of gain, character wise? The author ought to keep their characters consistent with the way they wrote them and how the story progressed thus far.

In this case, there wouldn't be any way for Eren not to react that way after learning the true reason why he got the founding titan power. This arc was necessary to expose the true nature of the titan stuff, while exploring how Eren got his power in the first place, something that was always shown as an abnormal occurence with more and more oddity in the mix (Grisha missing from the very get go, Annie/Reiner/Berthold focus on him, the military police, the coordinate activation, etc).
Let's not forget that Eren was one of the reason why the scouts were trying to go back to Shinganshina to discover what kind of stuff his father has left to them in the basement. When Eren learns about Grisha "acting as the enemy of humanity", that's basically flipping Eren's mental state upside down. It is as if the MC of a story realized midway that everything he has done wasn't for the greater good, but for the absolute evil.

Besides, a lot of people still thought Eren is mostly acting on revenge and impulse, whereas he is pretty much a devoted boy with some emotional issue at worst. This part of the story made pretty clear that he never considered himself as special or a messiah. Instead, it was because of his power that Eren pushed himself so hard to the point he thought he could make a difference after what happened 5 years prior the start of the story. He was a man on a mission, but cannot withstand the idea of wasting people's efforts by his fault, be it Levi's squad death and so on. This was something that was kind of alluded before, but not exposed thoroughly. It isn't like his character changed or anything: this is something that ought to happen due to the circumstances of his powers anyway.
That's why Eren monologue before creating that crystal wall is very important for his characterization: "For once, please let me believe in myself". This demonstrates quite well what I've mentioned before: Eren wants to do something. Or rather anything to help, even though he is absolutely unsure about what he can do. And that uncertainty was always something that clouded his judgment on several occasions. So this scene address that lingering issue.

And no, I don't believe Eren's hardening ability is meant to be a reward, but rather a logical progression after finding out the truth about the titans. You can tell that titan shifters don't really come with a full package of ability and that titanization was most likely engineered in some fashion, proved by Reiss' goods. It would have been quite weird for Eren to suddenly have the ability to harden himself like that, when it is Reiner's titan specialty, while Annie's could do that to a much lesser degree.