If your goal is to do X in space, then mining asteroids might be a valid means to help you do X, but not an end goal by itself.
Most missions don't need a large supply of water, hydrogen and oxygen located in a random orbit around the sun. If you need them at Mars or Moon, then likely it's easier to find the supplies there instead of spending a large supply of fuel in order to move a large supply of stuff to that orbit.
If we'd need stuff at LEO, it might be that flying to an asteroid (generally far, far away from LEO) and then pushing it to LEO is more efficient than pushing it up from Earth, but it's not so clear.
In any case, this scenario of mining asteroids is very different, I somehow think that the grandparent poster meant mining stuff for us back at Earth.
Most missions don't need a large supply of water, hydrogen and oxygen located in a random orbit around the sun. If you need them at Mars or Moon, then likely it's easier to find the supplies there instead of spending a large supply of fuel in order to move a large supply of stuff to that orbit.
If we'd need stuff at LEO, it might be that flying to an asteroid (generally far, far away from LEO) and then pushing it to LEO is more efficient than pushing it up from Earth, but it's not so clear.
In any case, this scenario of mining asteroids is very different, I somehow think that the grandparent poster meant mining stuff for us back at Earth.