Why do plants and animals need to be fast growing in deserts

"Fast growing" is a manner of plants adaptations in order to survive in dry lands.

Some plants develop quickly from seeds buried in soil in the long dry period. Insects feed on plants,meanwhile pollinating flowers that will make thousands of seeds before the rain water to evaporate completely, thus ensuring a new generation.

North American Agava grows for 15 years before it blooms. When it rains, their flower  produces seeds rapidly.From plants' seeds  new plants are being developed, and the old plants die.

In general, plants lose more water through the leaves, but the desert plants have to hold water. Some have special leaves ,"water accumulating" .Other plants lose their leaves during periods without rain. Once rain water dries, it's leaves fall to save water.

All plants have to survive to dry periods and to animals that eat their leaves.

Most land plants take their needed water from the soil,  by roots. They need water to cook their food by photosynthesis. In leaves, some water turns into vapours, which are lost in the air. Always soil water goes up in the plant and goes out in the air, in this way. If plants are left without water, first wilt, then dry, and if not receiving water, they die.