1. Draws Bruce out of retirement - he feels amused and intrigued by her, and he wants to get back out there. Bane being a threat being the other aspect of that.
2. The interplay with her and Bruce and her and Batman is some of the best character work in the entire trilogy. So much goodness there.
3. Provides Bruce someone to be with and move on from Rachel for good.
That's just off the top of my head right now
I think he meant "story" wise, not literally in the plot. Like what she represents; to Batman, to Gotham.