Water is the sole component of wheel throwing, without it the task becomes extremely difficult. Adding a thin layer of water before adding clay to the wheel helps it stick. Too little or too much water will lead to the clay sliding off, which usually occurs during centering. Water lessens the amount of friction on the clay itself, but also adds to the static friction that keeps it to the wheel.
On the throwing wheel water must be applied whenever the clay or wheel gets dry as it can and will hurt the sides of your hands that are against the wheel.