Second, they need air, specifically carbon dioxide, which the plant converts into sugars, protein, and other building blocks through the energy derived from the sun. Lastly, plants need water for a variety of functions. Water absorbed by roots makes its way to the leaves where some of the water molecules are split during photosynthesis, releasing oxygen and energy. Water not used in photosynthesis cools leaves in a process known as transpiration. Drought stressed tomato plants will be smaller with fewer fruit and more likely to succumb to disease and insects.
Gardeners know that blossom-end rot increases with dry soils.
When water is applied to the soil, there are a variety of things that can happen. If too much is applied, some may runoff and never soak into the ground. This results in erosion and is certainly something we need to avoid. How much depends on the temperature, humidity, and air movement. A sunny, dry, day with a bit of wind will result in much more evaporation than a cloudy, cool day.