The valve is used to admit liquid to the hot end when heat transfer is needed. Water is often the liquid of choice as it has a high latent heat of vaporization, a desirable trait. One problem with water is that the vapor pressure is less than 1 atmosphere at room temperature. Other fluids are refrigerant gases and fuel gases. Butane, or pentane if you can find it, is preferred when using flexible lines as the pressures are lower.
Propane is preferred when solid lines are used as the pressures are higher.
Filling a heat pipe using the pressurized gases is easier than with water or other room temperature liquids as evacuation is not needed. I use a technique where I over fill the pipe and then carefully vent the gas. The venting expels the entrapped air and other dissolved gases. This venting process should not be used with Freons or other green house gases because it's against the law, besides it's expensive. The fuel gases are very cheap.