Also for periods of heavy battery use. My iPhone 6 gets roughly 3-4 hours of playtime on games (most recently Knights of the Old Republic). I'm not complaining because that's great for a phone today; but 8-10 hours of playtime would be nice a few years from now. I take a lot of long flights and it's nice not to have to carry a spare battery or plug into my laptop.
So we actually need a phone to last for 2-3 days of regular usage so it's up to the task of lasting a full day in the worst case.