At first it is not about metals, but water. Water is the oil of space. You can use it to make propellant, human/plant consumables, radiation shielding, and for

At first it is not about metals, but water. Water is the oil of space. You can use it to make propellant, human/plant consumables, radiation shielding, and for further chemical processing. Once you have cheap water in cis-lunar space, it becomes 'easy' to move payloads around the Solar System and start the process for other resource utilization.