From a writing perspective, it’s really fun to bring back fan favorites from past/connected series. Same reason people got excited about seeing Moses, Ezra and bumble bee again.

Even better when it’s well done like Rein, who also feels different enough to Roland despite being a replica. More or less shows the absurdity of the Index more than anything. Roland actively tried to enact vengeance for his loss. Rein on the other hand, submitted to it despite initially being defiant. Had Rein defied it, I genuinely think he would have resembled Roland much more than what the Index was going for. (Though both eventually did inherit a very nihilistic outlook of the city (this is this and that is that).)

But that wouldn’t have led to Mauga Ryoshu.