The water absorbed by the root hair cells passes through the plant in xylem tubes and eventually reaches the leaves. Energy from the sun is eventually used to convert water into hydrogen and oxygen.

Structure of the root hairs 

Hydrogen is combined with the carbon dioxide to produce the food (glucose) for the plant, whereas the oxygen, which is a by-product of the entire process, is let out through the stomata.
If a plant does not absorb enough water, it will wilt or go floppy. Without water, it may also not photosynthesize quickly enough, and it may die.

Roots
Root systems and root hairs are adapted to play a special role in the plant. The root network spreads out to absorb water (and mineral salts) from a large amount of soil. It is also adapted to hold the plant firmly and provide support (anchor) to the plant in the soil.