For the Best Battery Life
For many people, the relative capacity of one smartphone's battery over another isn't much of a priority, the assumption being that modern phones simply hemorrhage power at a constant rate, and should be recharged any time an outlet is in reach. Setting aside all the easy tricks for conserving energy — like turning Bluetooth and wifi off whenever possible — a bigger battery is obviously better than a smaller one, and might curtail some of the compulsive charging habits that actually shorten the lifespan of lithium-ion batteries.
In this regard, Apple is still playing catch up with some of its rivals. While the iPhone 6's reported 1810 mAh battery is a clear improvement over the 5s (whose capacity was 1570 mAh), and the 2,915 mAh battery in the 6 Plus is even better, the Huawei Ascend Mate 2 4G blows both of these models out of the water, with a mighty 4,050 mAh of capacity. Of course, how a given phone uses the energy it has stored is a major factor in real-world power management, but even Apple's own estimates of 12 hours of web browsing with the 6 Plus don't measure up to the Ascend Mate 2's 16 hours. And other non-Apple handsets also hold more total power, including the Samsung Galaxy Note 4 and the Sony Xperia Z3.