People are going to bog you down listing all the practical everyday uses for weapons-grade plutonium, but let's try a different tack. Stipulate for sake of argument that there is something dangerous with no beneficial civilian use, so you want to ban it. But there is public demand for the thing, or a lot of people just hate you banning things and will make every effort to thwart you.

Sellers split the item into parts so none of them individually is the completed item but anyone can buy them and stick them together. So you pass a law that says the "lower bit" is the part that's banned, and they can't sell that part. People start selling "80% lower bits" or whatever the nearest not-illegal thing is that can be converted into it. 3D print files for the part appear on the internet, or detailed instructions for how to manufacture one yourself.

What now? Ban 3D printers and CNC machines? Ban anonymous communications or encrypted communications with anyone outside the jurisdiction, so people can't distribute the parts files? Now you're banning things with beneficial public uses. But if anybody can easily reproduce the thing the thing then the ban is ineffective.