Some sportsmen concentrate on strengthening certain muscle groups depending on their sport.
Sprinters work on their legs.
But if they do not train their upper bodies they will not run as fast. Although your abs do not appear to be involved in karate, they are. You can move with increased balance and body control if your midsection is strong enough to steady your movement. Improving strength in your abs will help you change directions. Your back won't get sore after training.