I'm writing about people living in space who can mine asteroids and I'm wondering what minerals or other resources, if any, they might need access to larger planets for. Since planets and asteroids ultimately condensed from the same cosmic debris, I assume they would contain the same basic elements: iron, carbon, magnesium, aluminium, gold, nitrogen, etc. are all found in asteroids so you wouldn't generally need to visit a planet to find them.
So that said, what substances not found in asteroids (especially ones with a known industrial use) are synthesized in large quantities by geological or biological processes and might therefore make planets valuable to space-dwellers?
Earth's petroleum and Titan's hydrocarbon lakes come to mind.
(Assume these people have access to technology like antigravity or space elevators or something else that makes it relatively cheap to lift resources out of a gravity well.)