[Ravashak]

If there was a preset destiny for everyone, it takes away the responsibility someone has.
"Why would I care about the things I do, I'm meant to do it anyway, or I wouldn't be doing it" is a very silly/stupid way of thinking, but it'd justify all actions in the minds of those that are... less stable.

Even if there was a being / group of beings that controlled the fates of every being, unless they want to screw things up worse than they already are at places, it's important that we at least have the illusion of choice, which would also bring a feeling of responsibility, because it was their own choice (or so they think).