One reason is to work on an area of difficulty. This is similar to a musician learning to play a song. If only one part of the song gives the musician difficulty, then they would practice that part until it sounds better and then try the whole song. You can do the same thing with difficult parts of kata.
Another purpose behind repeatedly practicing a certain portion of a kata is to examine the bunkai more closely. This is an activity I do quite often when another Sensei or a high ranking student is running the warm-up exercises in a class that I am helping them teach. I find a certain sequence that I am either unsure of the bunkai, or suspect that there may be more to it than the bunkai I know (there's always much more to every technique than what I know) and I repeat it several times while visualizing different attacks until I discover an application that is new to me and effective.
A third reason is the development of power. Practice a technique dozens of times consecutively either on the air, on a target, or both to develop power by getting the technique just right, and the repeated action exercises the specific muscles involved in throwing the technique.