Water

According to Lao Tzu, water is like the highest good in three ways:

it brings life to all;
it seeks the lowest place for itself;
it overcomes obstacles by gentle pressure, not brute force.

When a child on a terraformed asteroid asks why water is important, he is reminded that:

he drinks it;
he washes in it;
it provides most of the oxygen for terraforming;
it is necessary for every industry.
-Poul Anderson, "Rambling With A Gamblin' Man" IN Anderson, Tales Of The Flying Mountains (New York, 1984), pp. 129-163 AT p. 133.

He also knows that water is the most convenient way to transport the hydrogen needed for fusion. But he nevertheless asks: why mine water on a Trojan asteroid rather than on closer-in asteroids, on Jupiter or on Jovian moons? The answers are that:

the Trojan asteroid has an ice core and rich lodes;
miners of the Jovian atmosphere seek "'...still more valuable materials...'" (p. 134);
the moons are in a deeper gravitational well and are too big to terraform so that working there would require expensive life support.

I never knew that.