Different time, different context. This would have been around 2010, when powering anything with USB was relatively uncommon. Some USB peripherals required a

Different time, different context. This would have been around 2010, when powering anything with USB was relatively uncommon. Some USB peripherals required a separate power supply or dual USB connections in order to provide enough power. Devices that charged via USB typically provided a special USB power adapter in order to facilitate faster charging. Even today, dual battery compartments have marginal utility. Since it is built-in, it offers extended battery life without carrying around extra modules. Spare batteries are mildly more convenient to carry around (no extra cables). Mostly though, it enforces user replaceable batteries.