As far as we know there is little that's economical to bring back to earth. There are quite possibly enormous iron and nickel reserves from meteor impacts (e.g. [1]). Mining them for use on earth isn't that interesting, but if you want to build large structures in space (or on the moon) having metal that far out of earth's gravity well is pretty valuable.
And then there are the unknown unknowns. We spend a lot of time prospecting earth, and haven't spent all that much prospecting the moon. This will be much easier with boots on the ground. And while the moon's lack of geological activity means it isn't good at concentrating resources, it also means remains from impact events remain much more reachable and concentrated than on earth.
Claiming that iron reserves on the moon are valuable because they make it easier to maintain control of the iron reserves on the moon isn't going to convince many people, and zero competent people.
Iron reserves on the moon / asteroid / space are only going to become valuable once there is a reason to mine, smelt and forge in space, outside of Earth's gravity well. So if we can ever get say giant space ships and space stations built in space, sure. But unfortunately, that looks unlikely this century.
Don't know why you are being downvoted... I've noticed that this whole asteroid mining (or really mining of any extraterrestrial body at all for that matter) has almost reached meme levels of hysteria here on hackernews... I too don't understand it.
It might not be economical to ship it back to Earth. But launching from the Moon is far cheaper than launching from Earth, so if there is ever a significant demand for materials in Earth orbit it makes sense to supply from the Moon.
This is so far off, though. Whatever raw materials you mine from the Moon need to be refined and then you probably need to manufacture something from them, with any other components of the manufactured product being imported from Earth. Not saying there would never be a plausible scenario, but it's so far off, with nothing even resembling a hypothetical plan at this stage.
The moon has helium-3 which would be useful for fusion. Getting it back to earth would be easier than getting off earth, you have the possibility of cargo rail guns which only needs materials to fire which you can source on the moon.
