Combinations of kicks and punches are a challenge because they require everything: speed, strength, endurance, balance, flexibility, suppleness, perceptual awareness and good recovery time. The reason they are so good for training however is down to a secret attribute few actually think about: mindfulness.
There is simply no way you can zone out and still perform at this level. When your mind is so actively engaged in monitoring your body and enabling it to move fast and accurately it acts with it as one. It is in the synthesis of mind and body that you get to transform into a real weapon.