Knives are useful. What's the the benefit of porn? Before I discovered porn, I just used my imagination and got the same level of "entertainment" as you now must get from porn.

Porn does harm some people, as it's easy to get addicted to it, and there's a lot of bad things happening on the supply side of it. I'm sure porn has also changed how people behave and what people expect in relationships, but it's hard to study such things. The amount of sex we have has decreased recently. I don't know if it's due to porn, but it'd be worth studying. If sexual satisfaction is easily available, there's less reason to seek the real thing.

It's beyond parents to fix these things, at best they can delay the discovery of porn for a few years, but that doesn't fix anything.