asteroids are suitable for space based industrial production

it's easier to extract material and gases from asteroids than from the moon or mars, 

since asteroids have insignificant gravity and moon and mars have enough gravity that energy needs to be expended to leave the moon or mars 

industrial equipment that functions in space is needed, 

robot arms with polymer 3d printers and metal 3d printers, 

in the beginning technology and equipment will be transported from earth to space 

and materials from asteroids in the asteroid belt will be used to build structures 

later when suffiently large structure are built in space, 

then technology and equipment can be produced in those structures 

air for breathing and agriculture will be extracted from asteroids, 

water will be extracted from asteroids 

systems of space based industrial production supporting clusters of space stations is more efficient than expending energy moving material and products between surface of planets and space