I realize these sections are meant to display the female dancer's grace, line and balance. But isn't such a display redundant considering all the abilities she has already shown in allegro?
Here is the problem. You are watching it as though it's purely or primarily a sport. As though the classical pas de deux's only context is, as you said, to "display the female dancer's grace, line, and balance" as a short program or routing in figure skating or gymnastics would. But this not the only context - in fact, it can be negligible when appraising the work as a whole. Ballet is an art form. The dancers move the way they do in classical PDD not only to "display" themselves but to tell the story (often the entire process of falling in love in 10 minutes or less!) and illustrate the music in a way that they have been trained to do since childhood. When you get to a certain echelon of dance ability, strength and flexibility are more or less taken for granted. You begin to observe the details - the "fragrance" of the movements, all in millisecond variations of timing, incremental differences in angles of the joints that give vastly different emotional impressions. The use of the eyes and face, fingers and wrists and feet. You will learn to watch feet in a way some might consider creepy.
You cannot watch ballet as sport - not because it isn't athletic enough to be one, but because to do so is to miss nearly everything worth seeing! Watch a ballet from beginning to end, one with a strong emotional/narrative element. Attune yourself to the story, music, aesthetic richness, acting. Like you would an opera, musical, painting, poem... anything but sports!
Right now you are only seeing the very tip of the very surface of the iceberg. I hope that I and other users here can help you get into the right mindset to appreciate this beautiful art form. It is so very worth it, and not as hard as it sounds! It will click for you.