The fundamental purpose of drills, i.e. paired practice using kata, etc., is to learn the fundamental principles of martial systems so when the time comes to actually apply technique in self-defense the practitioner will have encoded the principles as applied in karate. This is why this type of training is fundamental or basic to martial arts, i.e. it is a primer to learn said principles so they will be available to the lizard brain when a less basic training and practice are started. The kumite drills help to teach the student about control, natural action, reflexive action, breathing (breathing is often just assumed when it should be a stronger focus in martial arts), posture, spinal alignment, axis - both major and minor, structure. heaviness and relaxation, wave energy, centeredness, triangulation points, body-mind cohesion, etc., void as it applies in a physical sense, centripetal and centrifugal force, sequential locking and relaxation (this is a beginning of the chinkuchi effect), peripheral vision, tactile sensitivity, and rooting.
There are more principles involved but many of those are incorporated once the fundamentals or basics of martial systems are ingrained and encoded into the mind-state, i.e. where the instinct allows the lizard brain to draw from them to apply as needed in self-defense.
There are some assumptions as well that while learning through basics, kata and kumite (includes drills) that one has gained the knowledge in proper application of martial arts if studied for self-defense. As a combative art form it is understood that combative is a step above fighting and both are illegal in our modern society. There are specifics that must be understood so that when studying a martial art as a means of self-defense that are complicated and often convoluted. The Sensei will recommend materials that must be studied so when the time arrives later it can be applied in proper training and practice.
Drills, kumite and kata must also be understood are NOT means to self-defense directly but tools to get a martial artist to that end goal. There are avenues that must be taken in order to make it work in reality as martial arts training alone will always fail unless the other connections are made and encoded. An example is training to handle the stress induced adrenal chemical effects that violence, etc. will cause. Another example is understanding force decisions, the legal ramifications and the emotional and economic results of self-defense and fighting (when SD crosses the line into fighting, etc).
Remember that kumite or drills are merely that stepping stone one steps across to move from inexperience toward actual self-defense and its applications. As an art form outside of SD, etc., it is still a stepping stone from the model of “shu-ha-ri and/or shin-gi-tai” concepts that all martial artists must learn regardless.