The structure of a root hair cell differs from other root cells in that it has a long, thin extension supported by the central vacuole, which greatly increases its surface area. Like other root cells, it has a thick cell wall, huge central vacuole and is separated from other root cells by a thin layer of cytoplasm. These plant cells are essential to obtaining water from the ground for photosynthesis.
Plants require water for several purposes, most prominently photosynthesis, which they use to generate both food and structural materials for growth. Water does serve other purposes, however. Every living plant cell, such as those in its leaves and shoots, maintain their rigidity through water pressure, using the tension from their large central vacuoles on their tough exterior cell walls to maintain their shape like a Mylar balloon does. It also helps cool a plant by evaporating water from tiny holes on the undersides of leaves.
Photosynthesis and cooling both use water rapidly, however, so plants require a constant supply of water. Root hair cells help supply sufficient water by growing large, thin extensions, which increase their surface area dramatically. This increase in surface area allows them to absorb water far more rapidly via osmosis. The extra surface area simply allows more places for water to diffuse across their membranes.