Having a backup light allows for two different styles of light to be used.
The best way to carry a spare cell is in a spare light.😀
QFT!
I got a holster with a S&W Galaxy that has a pocket on the side for 2 spare CR123s (of course, I don't
use 123s), and just recently got a light that fits it nicely, with an 18650 in the side pocket (too tight with the Galaxy). So that cell gets carried separately, as do any 14670, 14500, and AA/AAA spares, but I still carry more 18650 flashlights than I could reasonably need as true backups (in case my main light dies), so that I have enough cells to keep even the bright one going for quite a while, and so that I have other options.
I've never seen one light that could throw like my WF-008, flood like my Download MC-E, blink in any color like my First Alert 10-color (3xAAA modded to 18650) and have a red laser, white LEDs, and UV LEDs like my DX 3-in-1 (3xAAA modded again, and serves mainly as a battery carrier), but with three lights, I've got any of those options available in a half-minute for as long as
any battery has juice left, and I have them available in seconds in most cases. (It's not uncommon to swap a spare into the MC-E, but the rest rarely see 20% usage a day.)