Great question! The answer is that plants need to move things up from the ground and they don't have a pump like our heart.

Leaves have small holes in them that can be open or closed. This allows them to take in CO₂, which you probably know is needed for photosynthesis. It also allows them to get rid of oxygen made in cellular respiration. Water evaporates through these holes. As the water evaporates, more water is drawn up to replace it. Capillary action is the name of the process that causes water to be drawn up into small tubes (like the ones in a plant). It happens because water molecules are attracted to each other.

So this flow of water up the plant happens because of the evaporation. The water can then go wherever it's needed and it has nutrients the plant cells need dissolved in it. A lot of water can be lost to evaporation, too, which is why they need so much.

Not all plants need a lot of water. Plants that evolved in the desert or climates with dry seasons, have adaptations to help conserve water. Can you think of some?

You may want to explore a career in plant ecology or plant physiology.

Thanks for asking,

A lot of the water we give to plants either evaporates or sinks too deep into the soil for the plant to get to, so we have to give plants extra to account for this. Still, only about 5% of the water in plants is used in photosynthesis. The rest generally evaporates from the leaves to keep the plants cool. This is essentially the plant's method of sweating to maintain its temperature. Almost all of plant water intake is used in this.

Plants move nutrients through their bodies through a process called transpiration - the plant evaporates water from the leaves, and surface tension of water draws up more water from the roots, entraining the nitrogen, phosphorous, and everything else the plant needs to grow within. However, by evaporating water, the plant needs to get more in order to continue doing this. It's basically their blood.