Mining is the big reason to launch mining robots into space. For now though, there isn't anything we can economically mine.

Mining helium-3 from the moon has been suggested. It could potentially be used for fusion power. But we currently can't make a helium-3 fusion plant, and even if we could, it would have to compete with cheaper, more abundant fuels for fusion. So it's far from certain that helium-3 mining the moon will ever happen.