Falcon 9 booster preparing for reuse testing at KSC

With asteroid resources, you can fly propellants and materials to anywhere in the solar system very easily. This is because they have very shallow and gentle gravity wells. Earth, Moon, Mars, all have deep steep gravity wells, which you can't leave with electric propulsion and you need a lot of delta vee. Bringing things from Earth just to low orbit means 98 units of rocket and 2 units of payload.

Imagine traveling with a credit card instead of having to bring everything bottled, canned and packed up from home.