Fuel
Split water and you hold hydrogen and oxygen, the propellant of the solar system, refined where it is found instead of hauled from Earth.
Life
Everything humans do in space runs on it: drinking, breathing, growing, and shielding crews from radiation on the way out.
Gravity Is the Tax
Every kilogram lifted off Earth pays a brutal toll to reach orbit. Every kilogram already up there is a head start no launch can match.