How does having a large central vacuole help root hair cell in its function?

When a cell has a large central vacuole, it has a lot of water in it. Because of hydrogen bonding, water gets 'pulled' from the soil into the root hairs, up the stem, and then evaporates on the leaves. As water evaporates, more is pulled into the roots. Having a large central vacuole full of water helps attract water molecules from the soil into the root.

Water is drawn into the root hair by osmosis (not hydrogen bonding). On account of the presence of sugars and other metabolites, the water potential of the sap is lower than that of the surrounding soil water. Water molecules therefore pass across the cell wall and protoplast, into the central vacuole. Thus, having a large central vacuole allows root hair cells to compartmentalize water uptake.

FYI: From the root hair, water passes to the vascular tissues in the center of the root via intervening phloem cells. Once inside the vasculature, hydrogen bonding drives water transport up to the leaves where it is lost as water vapor through the stomata in a process call transpiration (not evaporation).