I'd call AI-generated mods one of the best applications for AI-generated voice samples. It is extremely unlikely that a fan-made mod is going to ever get the original voice actor onboard, and without those voice actors, any mod necessarily cannot fit in with the rest of the game.
We could have a world where modding just doesn't happen, in general. But if we're going to have mods, that voice synth makes it practical to extend games that otherwise could not realistically be extended by third parties in a seamless way.
It's possible to make textures that fit in with original environments, or to model things that do so. Or to write text. But people are pretty good at distinguishing between voices, and so without the ability to do computer-synthesized voices for mods, one can't really create modified speech for existing characters.