Karate bunkai is the very reason for the existence of kata. It's the purpose of Karate kata. A kata is a collection of fighting techniques against common habitual acts of violence. Therefore, Karate kata teach us the oyo (application of techniques) to defend ourselves from common attacks. Having a set of movements ingrained in us through regular practice of Karate kata will help us remember these collection of deadly fighting techniques, provided we know correctly what each movement means. A powerful Karateka would have mastered most of the deadly Karate oyo in certain kata.
Kata are designed to hold the pertinent information on fighting instructions or Karate bunkai. Looking at any kata, a Karateka will find these universal elements of Karate: Usage of different stances, different hand and leg techniques, varying attacking power, a number of Kiai or shouts, attacking at different heights, utilizing body shifting, foot shifting (sliding), shuffling and moving in different directions. It also deals with hand and leg techniques that can be interpreted as attacking specific vital human pressure points, using pressure points strikes or better known as kyusho jutsu. Unlocking the kata will give you many different levels of Karate bunkai, from simple restraint to causing serious injury or even death.