Katas purpose in my mind atleast is to show one (if not many) combos or ways to string moves together.

Its like a dance. You can learn every single individual move in all of the martial arts but they mean nothing with out flow and being able to string moves together. Now akin it to dancing. A dancer starts out learning the basics and the classical dances. But truly skillful dancer uses what they know to make up new dances within their move repertoires and even have the insight enough to make new maneuvers all together. Also think of them as a centering exercise, you know the moves and how they should be preformed and it becomes an exercise in multiple move repetition much like you will repeat striking a certain method hundreds if not thousands of times in order to strike better and harder, thats how in my mind you practice combos.

Although I do think it is far more practical to spar, a limited number of opponents tends to narrow ones style to what works best against those few and if a kata id a proper one it should contain every maneuver used by that particular sub style you are practicing. Think of it as learning your ABC's before you start spelling and writing sentences.