Asteroids and comets will be valuable in so far as they contain plentiful minerals that are already in space. If people want to build bigger things in space the

Asteroids and comets will be valuable in so far as they contain plentiful minerals that are already in space. If people want to build bigger things in space then having available materials already in space to build them with would be valuable, and it would (probably/eventually) be cheaper than flying them up. Not to mention more environmentally friendly as mineral processing is a pollution intensive industry.