just 3 things
1. better battery runtime: would be awesome if every major OS update added more battery life instead of slightly decreasing it. Considering that batteries are all consumables, if you somehow optimized the software to be more energy efficient you can offset the noticeable decrease in battery runtime as the phone ages. Resulting in less battery servicing and less e-waste overall.
2. faster performance: this is kind of related with #1, Apple claims they don't intentionally slow down devices with major software updates, but they don't deny they do so unintentionally. It doesn't take a computer scientist to figure out new, poorly optimized software runs slower on older hardware. Have you ever seen comparisons between older iPhones running their original out of box iOS vs. the latest supported one? The difference in speed and responsiveness are huge.
3. improved usability: as phone screens are getting bigger, iOS has done little to optimize the UI and UX to better take advantage of the larger screen size. Another issue is the phone simply becoming unusable one handed since they place important UI elements on top. I think most of the usability issue stems from software, and the real challenge is how do you make interface easier to use w/o making it ugly?