Our core is the most important part of the body especially for taekwondo athletes as our movement involve the core. The power of the kicks is generated from the core to the leg and not just from the leg. If you think that the power of your kick is from the leg then you do not understand the technique well.
Core training is a very important or should I said vital as many of out taekwondo movement generated power from the core. Therefore, if you have a week core then you would notable to generate much power to be directed to your kicks. Also eventually you will suffer from back pain.
The core needs to be trained in order for us to be transfer the power to our kicks.