What is the advantage of stranded wires?

Flexibility is the main advantage of stranded wires; however for high frequency AC (i.e. radio frequencies), stranded wires increase the surface area, minimizing the skin effect (i.e. high frequency current avoiding the core of a wire, increasing its resistance at that frequency). This is not an issue at typical power frequencies (e.g. 60Hz) as the skin effect is still insignificant at those frequencies.