The plank, and its many variations; bridges, moving planks, diagonal planks, statics, etc., are all excellent exercises for Muay Thai and combat sports.
The plank is an exercise that works and develops the muscles the entire trunk, and teaches the fighter how to brace, and contract their abdominal muscles, as well as posterior chain to the max. If performed correctly; sucking the belly in and bracing, squeezing the glutes, contracting the pectorals, not allowing the scapulars or lower back to sink while trying to drive the elbows together, it is truly one of the best exercises that can be performed for core strength development. In short, planks challenge all the core musculature.
With direct relation to Muay Thai, a stronger core means harder kicks and punches, the ability to handle more body strikes, better footwork, and improved clinching and posture (staying upright in the clinch, and not being pulled down with ease).