- Better rendering of hues. This comes in nice for things like syntax coloring, or photo editing. The eye is evaluating a color next to the absence of light, instead of next to washed out light.
- Less eye strain when ambient light is insufficient. Ideally the brightness of the screen should be within the same range of brighness as the surroundings.
In my home office, I have about 20,000 lumens of light shining in a relatively small space, at a temperature of 5000K. In other words, it's a sunny day in my office regardless of the weather or the time of day. A quick and easy way to figure out if you have enough light (or too much) is to step back and point your cell phone camera at your workspace. Take a photo without HDR mode. Can you make out details of what's on your screen AND its surroundings? For most people, the screen will be washed out because the ambient light is way underpowered. Dark mode is a good response to that situation.
With quality and variable lighting, you can have the best setup: use light mode during the day; switch to dark mode when it suits you, whether it be for a task that works better in that mode or for evening time when you prefer to start sending a signal of sleepiness to your body.