Functional training means training like you move in every day life. To boost your metabolism and your athleticism plus prevent joint injury you want to train your body in all planes of motion. Including sagital, front to back, frontal, side to side, and transverse which is diagonal.
Rotational movements which are diagonal movements mimic every day twisting and are present in most sports including: swinging a bat, throwing a ball, or swinging a golf club. Forget sports, we rotate all the time in day-to-day life, including putting on a jacket or checking for oncoming traffic. Training in this rotational plane focuses on the obliques. The obliques and the core are the axis of all movement in your body. If you want to be your strongest and most powerful you will focus on this area. You want strong obliques that aid in twisting but prevent over twisting to avoid injury.