The question is what percentage of your time does one operate in a state of “joriki” in all that you do in life? Training is to facilitate being able to place oneself in such a centered state so we are “in the zone” whenever we choose, rather than just on the rare occasion. In fighting, one will have to switch this "state" on in an instant, and do so in a high pressure and potentially emotional situation, to allow one to clearly and without emotion focus on the options and the opponent. Kata practice is an isolated way of training the mind in concert with physical actions to be in such a state and provides a tool which lacks the complexity of an opponent. This allows one to fine tune the skill - after all, if one cannot do it alone, it will not be achieved in the heat of a street fight.
With regards to kata performance, hopefully at some point in your training you have performed a kata that you were very happy with in such circumstances as a belt test, or competition, and in retrospect you realized you were not thinking of what you did as you performed (mushin) it, you found that all flowed with balance and power, no realization of time were present, and nothing could distract you (joriki).
All the above relates to skill development not only for the world of combat but for personal development in every day life i.e. the way one carries themselves in any stressful situation, or day-to-day activity. All the founders of the current major karate styles stated quite clearly in one form or another: karate is all about character development. An excellent text on the topics of Mushin and Joriki can be found in “The Zen Way to the Martial Arts” by Taisen Deshimaru and Japanese martial artist and Zen master.
"To overcome others you must first learn to overcome yourself..."
On top of our mindset, we of course also need to be able to use the techniques we have trained well, choose the right technique for the right time/circumstance (for street scenarios, I have been heavily involved in an evidence-based approach to technique selection pulling from emergency department and police statistics to be best prepared to avoid injury. See article/video at: Karate technique selection & Street Fighting Statistics & Medical Outcomes).
Related Article: Mushin...