Having an external battery pack can definitely increase brightness and runtime options, but single cell lights can also work for lots of people.
Lighting requirements vary a lot with speed and terrain. And I think there is additionally a relatively large range of acceptable brightness because of our eyes logarithmic sensitivity to light; even a 2x increase or decrease in brightness does not seem like a huge change.
Here is what works for me riding on curvy off-road trails in the woods of Ohio. My speed varies from roughly 5 to 20mph. You can read about my lights here. My handlebar light has a wide beam and about 6kcd. My helmet light has about 20kcd. I estimate my handlebar light is useful for illuminating a wide area up to 5m away which means the light puts about 240lux at that distance. And I estimate when I’m going fast I need to look 10m ahead on the trail and my helmet light puts about 200lux and illuminates a 6ft wide area at that distance.
According to this site:
200 lux is between a very dark day and an overcast day. Or about the level of lighting in a coffee break room or to do easy office work.