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.
What is a benefit of a video game? People can get addicted on gaming, let's ban it. What is a benefit a of a good steak? It is not healthy, let's ban it.
You don't like stuff other people enjoy and then you will go out of your way with ludicrous excuses to destroy it for others. Like a religious bigot.
Well sure, there's entertainment value. But there's plenty of harmful things that people enjoy that we ban because the entertainment value is not worth the downsides. I enjoy driving very fast, but I'm not allowed to do it (except in very limited areas) because the negative effects outweigh the positive ones.
Maybe problem is not a knife. Maybe problem are parents who are expecting that state will baby sit their children.