Here's why:

* Water is a reactant: Water is one of the key ingredients in the chemical reaction of photosynthesis. It provides the hydrogen atoms that will be used to create glucose, the sugar that plants use for energy.

* Water transports nutrients: Water carries dissolved minerals and nutrients from the soil to the leaves where photosynthesis takes place.

* Water keeps leaves hydrated: Leaves need to be hydrated to function properly, and water helps maintain the right environment for photosynthesis to occur.

While plants need water, they don't actually absorb it during photosynthesis. Instead, they absorb water through their roots and transport it up to the leaves. Then, the water is split during photosynthesis, and the hydrogen atoms are used to create glucose.