There is also another often overlooked benefit to asteroid mining: that the same technologies used to mine an asteroid can also be used to save the world! It is a well known fact that a large asteroid impact wiped out the dinosaurs. If our species is to avoid a similar fate we simply must have adequate resources in place for early detection of asteroids on an impact orbit with Earth as well as the capabilities to go to that asteroid and alter its trajectory so that it brushes past the Earth.
If the asteroid is caught early and it is far enough away it can be moved onto a safe trajectory for a relatively small energetic cost.
If we had an available large miner, we could dispatch it to the threatening asteroid and because of on-board fuel would be able to fire its engine and move it. Or else if caught well in advance the asteroid could even be mined on one side to change its spin and alter its trajectory in this manner. All this should be being done already as it is vital for the continuation of our species but there has so far not been an economic incentive to do so, this is no longer the case.
In searching the skies for mineral wealth we will chart the orbits of every sizable asteroid that poses a major threat to the Earth. Not only will we be vastly enriching the Earth and advancing human civilisation into Space but we will also be removing one of the major existential threats to all life on Earth in the process.