Free will. It’s supposed to be one of those Big Questions™, isn’t it. It’s supposed to be the solution to the Problem of Evil (ie. Why Does God Let Bad Stuff Happen?), but it’s not often explained how this is supposed to work. Science, with its atheistic, mechanistic view of the world, is repeatedly said to have disproven it. Philosophers argue about whether there’s even any coherent definition of it at all, or at least one which can help with what we’re trying to achieve by using the concept.
I’ll take these one at a time.
As for theology, it’s a simple matter of fact that there’s no such thing as God and never was, so the Problem of Evil is really a non-starter. But so much has been said on the matter that would be thoroughly wrong even if there were a God, and it can occasionally be helpful to speculate about what one might or should be like. Omnibenevolence, after all, is hardly a bad model for us if we’re trying to be merely benevolent.
The basic idea is that if there were no evil in the world, we would have no free will to choose good instead. This applies both to “natural” evil, or misfortune, and to “human” evil, or what we usually think of when we see the word. No natural disasters means no heroes to save people from them. No disease means no superlative doctors. And so on. And if nobody ever did bad things, not only would it mean the rest of us couldn’t distinguish ourselves as Better Than Them by choosing not to do them, but it would also mean that nobody could have even freely chosen to do them. Not only would there be no point in recognising or rewarding good behaviour, but there’d be no intrinsic merit in it either. You can’t hold a person responsible for their actions if they couldn’t have done otherwise.
Incidentally, the usual interpretation of Jesus as partaking in his dad’s omnibenevolence paints him this way. If a being is omnibenevolent, that means that they always, by definition, must do the right thing. They do effectively have no choice in the matter. They have no free will, and therefore when they do the right thing, there’s no merit for them in doing it. If Jesus is an omnibenevolent god, then his sacrifice at the end of the story is meaningless even if you ignore the part where he gets better (spoiler alert). Same goes for big-G God himself, for the same reason. A mere mortal who does even the slightest shred of good off his own bat is more worthy of praise than a divine creator who, if you believe the stories, does all the good in the world because he can’t help it.
But that’s all nonsense, really. If a real person maimed, infected, and killed his children on a regular basis, in the name of seeing who among them was really virtuous, we’d lock them up. If a real person had the power to heal the sick with a snap of his fingers, we’d get him to do it — and if he’d been the one making them sick in the first place, we’d think him a monster, no matter what his reasons. There are real people who advocate a complete absence of laws, or at least of their mandatory enforcement, but the sane majority of us rightly consider them to be loonies.