Something that is sustentative helps to support life or keep something alive.
Sustentative things are very important for life and survival.
Examples:
Water is sustentative for plants.
Food is sustentative for people.
Air is sustentative for animals.
A good night's sleep is sustentative for a happy day.
Love and friendship are sustentative for the heart.
In the desert, finding water is sustentative for survival.