Less voltage drop for a given load in #12 compared to #14, plus isolation of the (relatively higher current) receptacle loads from the (more easily perceived) lighting loads.
I don't notice, for example, if the voltage to my computer drops by 10 or 20 volts. It could care less, I could care less, it merrily computes away. If it falls below 100v or so I'll hear a click and beep when the UPS switches over, but that's it. But if I plug my 11 amp Hoover vacuum into a receptacle connected to my bedroom lighting, the voltage drop that the Hoover creates I will also see in the dimming lights. In contrast, by putting all of the lights on a circuit together, separate from the receptacles, the only voltage drop the lights see is what is common to both separate circuits, and that's the voltage drop in the feeder back at the panel.
