Liner socks help to keep the feet cool and comfortable. Wearing a liner sock under your normal sock will greatly reduce the chance of chaffing and blistering. The two fabrics of sock rub together rather than fabric on skin therefore minimising the chance of blistering. The liner sock also wicks moisture away from the foot, to the outer layer, keeping the foot cool and dry.
In cold weather Liner socks greatly increase the temperature around the foot by trapping a layer of warm air. An absolute necessity if the weather is cooler.